Best way to connect 2 preamps to 1 power amp

I've used the method suggested by Rwwear and it works great. Given it costs you nothing, it should be the first thing you try.
Assuming your tube amp doesn't have HT passthru, you simply connect the Front channel outputs from your HT pre/pro to one of the line level inputs on your 2 channel preamp. Set the volume control on the 2 channel at 12 o'clock, or some other easy to replicate position, and calibrate the output levels of your pre/pro at this setting. Then when you want to listen to HT, simeply power up your pre/pro, select the pre/pro input on your 2ch pre, and set the volume level.

I've since purchased a tube preamp with HT passthru - which is nicer, but I suspect you'll find the above method very satisfactory.

Man do you have this wrong. Your main preamplifier, I presume is the tube, you want to drive the fronts for surround sound via the Lexicon, therefore, you take the left and right surround sound outputs from the Lexicon and run them into the tube preamplifier into an aux in. Then you set the level of the tube preamps volume control with an SPL meter until all channels are at the same level, you then mark this level on the tube preamp as a reference level and everytime you place the Lexicion you must switch the tube preamp to aux in and set the tube preamps volume to the reference level you've noted before and voila you have a two channel tube preamp coupled to a Lexicon for surround.

Why not Email Bryston Canada and ask them about using the switch on the back for bal/rca. I'm sure they have had this come up before, I've wondered about this myself but have always used 2 systems.

Wow; you guys are awesome. I will try the suggestion from connecting the HT to one of the line-in of the tube. (I don't know I can do that). Also, I have some concern of switching over back & forth on the RCA/XLR as it will eventually become loose.
Thank you once again guys.

I also found out there is a by pass on the HT (Audio Refinement Pre-2DSP) whereas the tube doesn't (YS-Audio Symphonies Plus R). But I love the warmth on the tube. Is it of any use; please advice.