The only advice I can give is to get a sub amp with similar damping characteristics to your main speakers.
My speakers are run with a tube amp with low damping and my Bows are run with the Parasound which has a much higher damping factor. Although the Parasound is crazy good for the dollar there is a slight mismatch in the bass, and I mean slight.
With my experience, I would avoid mixing Class D and tube amps. This is just IMHO, the Bows are very sensitive to what you feed them. Slap them in the system and they will sound good. Experiment with cabling, feet, get the level just right, and have a well matching amp then it will sound great.
Because I haven't tested my Bows with a tube amp I can only hypothesize.
My future step will be to move my EAR 509 mkii monoblocks to the Bows and go back to a SET amp for the speakers. Overkill? You betcha!
Overall though, the Bows were some of the best dollars I've spent in audio. I love my Orpheus and Bows and they continue to get better as I upgrade other parts of system. I have never heard better bass integration.