combining amp and receiver for HT

What you are doing is fine. Is the HK a Home Theater receiver?? If so, a good sub like a REL would be a great addition. The High Pass connection can be connected to the speaker terminals of the Classe Amp for 2 channel audio, and the low-pass connection can be connected to a HT Receiver Sub output for movies. There are separate volume controls for both connections, so you can set both and forget it. The RELs also switch automatically to the low-pass HT connection when a signal is received from the HT receiver.

If you want to add a center, the B&W CC-3 is cheap and works great.

If you get to sit in the sweet spot all the time, I'd continue to run the center in phantom mode, and get a sub. If not, I'd get the center first. RELs are great subs, but if you don't intend to integrate the sub for 2-ch music, you can get more HT bang-for-the-buck (no pun) than the REL. I'd particularly look at SVS products. The company was started by two enthusiasts/addicts, and they offer a lot of value and performance. And if you are handy at all, a sub is not a terribly difficult DIY, and the result can easily compete with anything at 2 or 3 times the cost.