From Pass X250,5 to 350,5 : what to expect?

Better dynamics. I strongly disagree that more power is a bad thing. Unless you only listen to quiet whispy music more power will give you better dynamics which equates to sound that is more like real life.

I use Revel Salons.

You need sufficient power for dynamics, but your speakers determine whether or not a certain number of watts is a lot or a little. More power tends to corrupt the signal with more wire, parts, and complexity - so if you have sensitive speakers with benign loads you can take advantage of the inherent quality of lower-powered amplification. But yes, you do need enough power - no question, but that is a relationship between amp and speaker - with my speakers, 30 watts is a lot of power, with some speakers 100 watts is too little.

250 watts and the current drive into low impedances would seem to be plenty of power to deafen you with the Revel Salons, unless you have a really big room. I would think 110db would be plenty loud, the 350 would only increase the output to 111-112db, I would think you would be running out of the room before you could tell the difference. Again this is assuming the 250 and 350 have the same basic circuitry, dam[pening specs, and drive into dipping impedances.

Given the impedance curve and sensitivity of the Salons, you have all the power you need.