Best way to connect 2 preamps to 1 power amp

The best thing would be if your 2 channel preamp has a HT or Processor pass through. The other thing that will work is to connect the front channel outputs of the Pro Logic preamp to one of the unused inputs on the 2 channel pre. The only downside the the latter is having two volume controls inline.

Davidcpl: I'm not aware of any "high quality" RCA / XLR based switchboxes on the market.

As for your Lexicon meeting the above criteria, your best bet would be to contact the manufacturer and ask them if they see a problem with what you want to do. They would ( or at least "should" ) knot better than anyone else what will / won't work with their gear.

Other than that, Rwwear's suggestions are quite valid and may be the easiest to work with. I had forgotten about an HT pass through, but had assumed that you're 2 channel preamp did not have one since you were asking this question.

The other suggestion about feeding the left and right channels from your processor into unused outputs of your tubed preamp should work quite well. You will have to fiddle with the volume controls a bit to find a happy medium, but once you get the hang of it, this would be the easiest route to go. This would change the sonics for the left and right channels when listening to movies, but it shouldn't be anything to drastically worry about. My guess is that you're most concerned about preserving the performance of your 2 channel system, which this should do.

Kudo's to Rwwear for suggesting something that should have been both obvious and convenient. My desire to maintain "signal purity" at all times sometimes keeps me from seeing things in a practical manner. Thanks for keeping me on my toes and reminding me of this : ) Sean
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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.