The relationship between watts and sound volume is not what most people think. A 6 watt amp is not twice as loud as a 3 watt amp--to double the volume, you'd have to have a 30 watt amp. Doubling the wattage, from 3 to 6, will produce practically no perceptible increase in volume.
And it does not take much wattage to get really loud. Guitar players are always looking to get the sound of a cranked amp at apartment bedroom levels. You can buy micro guitar amps now that put out half a watt. I've tried them--they are much too loud for bedroom levels. If you crank up a half watt amp people need to practically shout to be heard. You don't need a lot of watts to produce a lot of volume
It will make a difference, though, in distortion--if a 3 watt amp begins to distort at, say, 20 decibels, and maxes out at 30 decibels, a 6 watt amp will get to let's say 25 decibels before it begins to distort, and max out at 32 decibels. I made those figures up--they don't express an actual formula, but they express the idea. If Phoenix is touting a 6 watt amp, the advantage will be less distortion at a given volume level, not more overall volume. That's a good thing. But you get in most cases a much bigger effect on sound if you change the speaker, rather than the amp.
The demand for more watts is almost always associated with bass frequencies. Bass frequencies take much more power to produce, which is why the bass player always needs a much bigger amp. Typically the bass player has much bigger speakers and cabinets as well, which is what large scale trains will always lack. You just can't make a G scale train with a 3 foot by 3 foot cabinet and four ten inch speakers. Bass frequencies are the thing that's missing, inevitably, becasue of the size of the speaker and the size of the enclosure
Speaking personally, I find that even very low wattage amps tend to be too loud, and when Im running outside I have them all turned way down. The neighbors don't really need to hear my trains running.
So I'm not really worried about massive speakers or lots of watts.
Now I can report some results--I had a small scale railways sound card inside an Aristo slopeback tender. it sounded very good in there, plenty loud and not distorted. I put it just the other day into an LGB powered tender, and it sounds terrible--hissy and irritating. Same speaker, same soundcard. It's the enclosure. The LGB tender is much much smaller and is emphasizing the higher frequencies, so the overall impression is of irritating hiss. I'mnot sure what I can do about it, other than try another speaker or alter the enclosure in some way, maybe taking the weight out to make the interior volume bigger