I have been searching for a solution and so far, the best available at affordable (relatively) solutions seems to be:
1) Use a low cost AVR, such as AVR-X3800H or Cinema 70, and use the center channel to power subwoofer 2. I forgot to try that when I had the AVR, should have sold it a week later, after testing this method. I did try it with the AVR-X1800H but it does not have the center channel pre outs, only the front left/right pre outs so I only tested it in 2.1.
2) minidsp Flex HT.
3) Motu M4
The AVR solution may be best because, possible to take advantage of the DEQ feature, very convenient, easy to configure, and it can function as a full blown AVR/AVP, for me, it will be a standby unit that hopefully will never be called on HT duties.
2) and 3), and similar devices (focusite, may be a little more complicated in terms of volume control, but may be not, I would have to really spend time reading their manuals before drawing any conclusions.