I agree with Wayne and Natale. The Art Audio amps that are frequently used by Soliloquy at shows are very powerful tube amps and probably do a better job at controlling the considerable bass capabilities of the 6.3. As a powerful OTL design I would also think the M60 would be an excellent choice, and I think either of these amps would make the 6.3s sing and may provide tighter, cleaner bass than the Cary(also an excellent amp). I've got 5.3s and have had problems dealing with bass in my smallish, square room, and I found plugging the ports with a couple tube socks can help tame the bass in certain situations without changing the character of the mids/highs--just a little tip. Best of luck.
Tim
Tim