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.

I highly recommend a good center channel. The center channel see's more information then any other channel when running in 5.1. (I've upgraded this speaker 3 times) The sub is also a must have item. Your missing out on the LFE frequency's with out it. I here Rel is great for quality music mixing. I personnaly went with a Velodyne HGS series. Lot of bang. Can be had for 50% off retail at this web-site if you keep your eyes open.

REL makes two kinds of Subs. They both work great with both HT and music. The ST subs (Stratus, Storm, Stadium, etc.) are design primarily for audiophiles for music. The Q subs (Q100E, Q201E) are designed for HT as the primary use. The Q100E has a handy Slam/Depth switch it you want more slam for movies and back to depth for music. REL subs will not shake the house like some subs, they are more accurate. The choice for movies then is do you want an explosion to shack the room or sound more real. I have two systems; the Storm III with my audio setup and the Q100E in the family room for movies.