Theta CasaBlanca II is the champ IMO. It handles both HT and 2ch. superbly. Its upgradability is limited only by the imagination of those setting the standards. Any new standard is handled with hardware/software mods available from Theta as necessary.
I couldn't afford it the CasaBlanca (even used $5000) and got a Proceed AVP instead, but the Theta CasaNova was a close second. Both of these units offer really great performance, with few compromises, at a more affordable price ($3000 used).
Both are upgradable, the AVP via software, the CasaNova via both software and/or hardware. Both offer really great HT performance (maybe a tie), with most considering the AVP slightly better for 2ch. performance. The AVP has the pass through feature many processors lack as Kthomas has suggested. This is important is you ever get a CDP or a DAC that has better DACs than the pre/pro you buy, otherwise you are basically limited to the performance of the pre/pro DACs.
As mentioned, it all depends on how much you value HT over 2 ch. performance. Another factor in my choosing was upgradabilty. I refused to purchase anything that was not in some way upgradable. If you want really great HT, and you can stand compromises in 2 ch., look at the Meridian, Lexicon, and the others. If 2 ch. is a main priority (it was for me) then CasaBlanca or AVP (or even that PAV/PDST or whatever that combo thing is from Proceed/$5000 used). I too am happy with just 5.1, though the AVP is 7.1 capable I don't have the room for more speakers yet. My den is a den, not a movie theater--yet at least, my next den might be different though.
Another unit I understand worth a look is the Bryston SP-1 pre/pro. Though it does not have the video switching all mentioned above have, its supposed to offer top notch pre/pro performance at a lesser price point. (Don't know if its upgradable?) I don't use the switching of my AVP anyway. Another may be along shortly to give more on it.
Finally, I recommend you see a few threads that have been posted lately going over some of this stuff (I wrote this same thing a couple of weeks ago). They may not speak to your exact question, but may offer some insight. Also see the websites of Theta and Madrigal. Hope that helps.