I have to disagree with S23chang. The De Capo will work beautifully with your 75 w tube amp. Everyone repeat after me: "The-DeCapo-is-not-just-an-SET-speaker" Good!
I run my De Capos with a 150-watt solid state amp. They play as loud, fast and deep as you possibly expect from a small speaker. The only limitation to the bass and volume in my system is my neighbors, nor the speakers. I am constantly explaining to people that the subwoofer attached to my system is NOT turned on--that's the De Capos.
For what it's worth, I've heard wonderful things about the ATC's as well (although I haven't heard them anywhere); however, they are supposed to be fairly power hungry. I'm sure your 75 tube watts would work, given your music preferences. But the De Capo is a pretty special speaker (that's why everyone gets so defensive about it); and I would put it up against a similarly-priced ATC any day for resolution and pure musicality.