If the BAT degrades the sound of hometheater, you wouldn't want to use it for two channel either?....correct?
I would guess that Unity Gain is techically different than Bypass. IMHO, bypass would suggest a direct, alternate path from input to output, whereas Unity Gain is the normal path with zero attenuation/amplification of the source signal. This arrangement allows the downstream main power amp to "see" the same signal that the upstream processor will be sending to the other amp(s) for the remaining channels.
So while it's technically different, the outcome is practically the same. As Dave suggests, if the BAT is capable (transparent, resolving, etc) of producing high quality two channel, then it should be satisfactory in a home theater signal chain.
For disclosure purposes, I have a BAT VK-3i running my dedicated two-channel rig (BAT power, Magneplanars).
My understanding is that Unity gain, HT Bypass, Cinema Pass Thru, Processor Pass Thru are all different names for the same thing. Each manufacture has come up with a name to make it appear as they have a special input/output circuit. But from my research I have done they all pass thru without coloration and that you still use the volume on your pre/pro and that the unitity gain just allows you to hook up to the amp without Y adapters and the confusion that would present. I am saving to buy the ARC LS26 to place between my MX135 and MC352. Taking donations if any one wants to help?
I agree with Theo that they are all similar. The tube preamp is setting the volume at the point where no gain should be added to the signal. The only way you would have a true bypass is if the Preamp would not need to be turned on and the signal would still be able to pass thru it. The only tube preamp I know of that operates this way is the Cary 2002.
Setting the preamp @ unity gain should improve the sound, otherwise there is no since in having it.
I am using a theta carmen II, theta dreadnaught, meridian 568.2 and aesthetic calypso in my home theater system. I would be happy to answer questions about the set up if you pm me.