My list sounds like yours,

My list sounds like yours, Atlas, Kato, some BLI, Rivarrossi, Bowser, Athearn. The ones I own and recommend are Atlas (kato drives) BLI, Rivarrossi. The others I have seen and heard run often and would buy if the models were suitable to my modeling period and models I wanted Kato, Bowser, and Athearn. I am referring to more modern manufactured locomotives not the ones from the blue box era when it comes to Rivarrossi and Athearn and Bowser, think can motors instead of the old giant open frames.

So its still Atlas and Kato

Rivarossi is unknown to me. I have never owned any of them.  I am watching some loco's on ebay mainly just to see what they are going for. I have no idea how to tell what year they were made on most of them and so no idea what the inside looks like. I really want some room for speakers.  There are some models that the sound is just outstanding and some with the same decoder and not so much.

Comparative sonic performance

Dear ??? (Stevie)

The number of factors beyond just the specific loco model and decoder choice,

which can directly affect the end-result "sonic performance",
(as judged by ??who's ears?? via ???what audition medium??? and ???acoustic environment???)

are numerous and rather-difficult to control
although reasonable easy to identify...

Suggest using the MRH Search box, with terms "best speaker" , "installation", and "iphone speaker" for some examples of modellers who let "the sound" drive "the install"...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

Stevie, I will post some

Stevie, I will post some photos of the 2-6-6-6. The post was one I used to demonstrate my engine prep work. There is lots of room in the tender for speakers and you can purchase the model with sound already in it now.

These are very nice steam locomotives, here is a pair in action climbing a grade in excess of 2%.

Thats a long train

I think that is one way to see how good your trackwork is.

My steam roster has a few brass and a few MDC. I don't expect to add to it any time soon. Most of it needs work. I have an H-5 I want to paint in the glacier park scheme and put sound and dcc in it down the road. But no other plans for the steam side. 

Bowser SD-40

The new Bowser SD-40 seems to be pretty nice, I don't have one, but have seen one run on a friend's layout, and it runs and sounds great.

+1 for Bowser

Bowser's SD40-2 are Canadian prototypes (you didn't mention your railroad of choice), so keep that in mind, but they are awesome locos.  I have three of them, they run and sound great.

Preferred railroad?

I guess that would be BNSF, GN, NP, SP&S, Milw rd, Pretty much any road that would normally be seen around the Pacific Northwest. But I could also repaint some for a home road if I ever figure out what that is? Repaint switchers anyway.

In wiki not everybody was original owners for SD-40's.  I will have to figure out who bought them from who or who leased what.

Speakers

I have been using the search function and doing a lot of video viewing. There are plenty of speakers available and maybe a first step would be to have a few different ones to play around with. In most of the video's they tell you which decoder and sometimes which speaker was used. I am a little more than interested in the speaker end of things. I am going to get some typical speakers with plastic enclosures and then experiment with wood enclosures. Maybe different kinds of wood.

Best sound installation?

Please let write me my intentions of sound on a steam loco - and sorry, I don't have experiences in diesels. Please compare this picture.

If there will be enough room in front of a loco model especially in smoke box then I prefer to install the speaker there. The sound of a steam loco will be generated mainly by the stack!
Maybe that the decoder will not find enough room in boiler so I place it in tender and yes, then I need a lot of cables between engine and tender, in many cases until to ten. However compare my blog entry of SP class A-6 and especially this picture, how I solved this problem.

Loco and tender are connected by two bundles of each five very soft cables which are twisted and so they are very flexibly.
And if you would like to ask for my preferred decoders - TCS.

My speaker of choice

I have been using these in the last 3 installs and really like the sound.  You have to fabricate your own enclosure, but well worth it.

http://www.ebay.com/itm/181755978826?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Speakers

I wonder how much it affects the sound having all that open space in the boiler ahead of the speaker? My first guess is it should enhance the sound.

What do you make your speaker enclosures from?

I don't know if the market is big enough for manufacturers to spend a lot of money on research as to speaker designs and materials. The sounds today from even just stick in your ear headphones is pretty amazing compared to just a few years past. I have seen stereo speakers the size of a cube of butter that had base response so good I would never have believed it was from such a small speaker and enclosure. It was almost small enough to fit in a dummy loco. I am sure that could change if the market was there.

I think the biggest advancements are going to come from modelers experimenting with different applications and posting our findings on web boards like this one.

Another thing that interests me is the new Paragon 3 with the woofer under the layout. I expect other manufacturers will follow suit in some form. Old stereo equipment and large speakers are dirt cheap! I have several already. Why not several speakers under the layout and with a sensor that knows where the loco is and routes the sound to the nearest speakers automatically? Then the rumble would follow the loco as it should be.

.030 Styrene

I make my enclosures with .030 styrene.  easy to work with and you can fabricate pretty intricate enclosures to fit just about anywhere you have room.  As for sound from "ear buds",  the ear buds from a few years past were pretty much small speakers in an enclosure that sort of fit in the ear.  Most good quality earbuds now come with different sized silicone ear inserts.  This does 2 things: 1; the ear bud fits "in" the ear canal and 2; It seals off almost all background sound.  The pair I have are a "closed" design meaning that only the ear side allows sound out.  This is why there is much better sound and bass response from traditional headphones or older ear buds.  I don't believe they would be good candidates for locomotive speakers because they are designed to work at much less power than most decoders put out. 

Recent sound decoders

I like Loksound. You can update the sounds if a better version comes out, with different sounds or different functions. Plus they are easy to tune for slow running with an automatic tune function.

Tsunami are good too, if you like the sound and functions they have. You are stuck with the sounds on the board when you install it, but that may not be an issue if you prefer the sound. You have to do manual tuning of about 4 CV values to get slow running.

(And the Tsuname non-sound decoders are good too, just the same as the sound versions, just without the sound. Use these to mix non-sound with sound, for consisting.)

The WOW sound good, but I have not had any luck getting them to work after install. (May need more development.) Some folks love them. Give them a try, but carefully.

QSI/BLI hard to program. Even with special hardware. If you have one that works, you are ok. But fixing any bad setting, good luck. And the QSI will scramble if a short happens anywhere on the layout. (There is a reason they are not available anymore)

MTH - due to potential lawsuit, no comment. Get the DC version and add a real decoder. (They sure LOOK nice though)

MRC - They have been getting better. Cheaper than most others. Top notch speakers.

Digitrax - I have only one friend who still uses these. Cheaper than MRC. 

When getting Loks with sound already installed, check that they have the full decoder, not a knocked down version.