Smccull,
First, I have been going through the same scenario over the last 5 years and have had many different iterations of "combo" systems in my listening room. I speak from years of personal experience!
There are several different ways to go about adding HT to your system. It is all going to depend on your budget, how much hassle you can tolerate when engaging the HT portion of the system and how much sonic performance you demand from the HT portion of the system. I do like the fact that you seem to want to add-on HT and not try to mess with your existing 2-channel rig that you currently enjoy.
I did not see what main speakers you use when I viewed your "system". For quality HT, you will need (at least) to match the center channel speaker to the main L&R front speakers that you plan to use in the system. If you don't plan to do this, or if there is not a matching center channel speaker to your mains, you might want to consider not even bothering. Maybe just add a sub to your system and crank it up a bit when watching action flicks. Of course, you can always use "phantom" center channel mode (no center speaker, center content sent equally to L&R fronts).
If you truely like your current 2-channel system and want to add HT, the easiest and least expensive way is to buy a decent HT reciever with (at least) L&R main pre-outs. You can connect these pre-outs from the HT receiver to a processor loop/HT bypass on your pre-amp, or, if your pre-amp does not have a bypass, simply connect the L&R pre-outs to an unused input on you pre-amp. Now, you current 2-channel rig (amplification and speakers) will provide the L&R front output for the HT system. Connect center, surrounds, sub and HT source devices to the HT receiver.
As you can see, this setup does not have any affect on your current 2-channel rig and it does not even need to be turned on when listening to music. To play HT content, turn on the HT receiver, select HT source, select ht bypass/processor loop on the pre-amp (or select the appropriat input if you do not have HT bypass). You will need to "pre-select a volume setting on the pre-amp (like top-dead-center) if you don't have an HT bypass crcuit and you will need to "calibrate" the HT setup. You will always have to set the pre-amp to this pre-selected setting when listening to HT or you speaker output calibration will not be correct.
This is much simpler than it may seem from this post. Of course, an HT bypass/processor loop simplifies the process because there is no need to "pre-select" the appropriate volume each time you engage the HT portion of the system.
I have used a setup like the one described in several different iterations of my 2-channel/HT combo systems. That said, I have now abandoned the combo setup and have simply built two competely separate setups (one for 2-channel, one for HT) that both sit on one audio rack. I placed relatively inexpensive "on-wall" HT speakers above/beside my Plasma TV for L/C/R HT speakers. These speakers are up and out of the way, so they do not affect the performance of my 2-channel rig. This may also be an option for you and it would have the benefit of not causing tube life issues for you when watching TV/DVD.
Here is a link to a picture of my current setup:
http://forum.audiogon.com/cgi-bin/fr.pl?vaslt&1113439283&read
Enjoy,
TIC
