In response to the other's comments and your question :
1st off, I too live in a fairly 'dry county' when it comes to hi-end audio, so I have a limited base of equipment to listen to without driving 100 miles to audition. Are there better combinations then what I have - OF COURSE!! But - like Bob, I have to weigh everyones' opinion equally and realize that just because someone says they love or hate a piece doesn't mean it will or won't work in my environment.
-- to those that say 'forget MartinLogan' - that wasn't the opinion that was asked for. I have ML's and mine are certainly 'a pair speakers that makes you smile when you sit front it' to quote broken english. MartinLogan speakers are NOT toys if they are used with the right hardware and set up correctly (which can admittedly be difficult in some rooms). IN MY SYSTEM, IN MY ROOM, IN MY OPINION they work great. 'Nuf said.
-- I agree that Classe' would be a good amp product to investigate for use with SL3's, and -to me- represent a step up from Bryston for sound quality. Good stuff; their upper-end amps have a reputation of being on par with comparable Levinson, Pass, Krell...
-- Back to your question, Bob - I tend to do all of my critical listening at moderately high volumes, and the Aerius are probably more limited in their ability to resolve details at low levels when compared to the larger ML's. But, when I AM using them at lower levels, there seems to be a nicely linear response from the Pass amp. Not knowing what you would be replacing the SL3's with, I can't comment on X-250 vs. X-350, other then to say that the 250 should be very capable of driving any ML very successfully up to the Prodigy series. The 350 would be even more so, but may not fit your budget (or your equipment rack - that's one huge amp!!).
-- Every product line seems to have it's own sonic character, so you should probably spend time listening to some of these amps to see what you like before deciding.
-- Also, ML's tend to be very sensitive to what speaker cables are used, so if you are going to spend big $$$ for a new amp, I would suggest you invest some research time and $$$ on the cables as well. I changed from MIT Terminator's to Analysis Plus Oval 9's when I moved from Rotel amps to Bryston, and am actually changing from the AP's to Shunyata Lyra cables this week. The sonic changes from the MIT's to AP's was DRAMATIC to say the least. I can only hope the Shunyatas make another big improvement.
BTW - I have a GREAT, super-reliable contact for purchasing pristine Pass stuff at awesome prices. Email me privately if you are interested.