Best way to connect 2 preamps to 1 power amp

I have 2 preamps; 1 tube 2-channel for musics; 1 solid state pro-logic 5 channel for movies; both have RCA output. The power amp for front channel is lexicon 212 (Bryston 3bst); has both xlr & rca inputs w/ selector switch. Shall I:

1) use rca y-adapter for the 2 pre-amps to the 212
2) Use rca for the tube & xlr (w/ rca xlr adapter) to the 5-channel. Use the selector switch to change source.

Please advice any concerns/problems for any/both connections. Or any other better solutions.
You can NOT connect two preamps to a common power amp using "Y" cables or splitters. The preamp that you are using at the time will not only see the amp as the load, but also the other preamp that isn't being used. You will definitely hear the difference in sound, even if the unused preamp is turned off.

You might be able to use the RCA's on one and the XLR's on the other, but this would depend on the design of the amp itself. If the amp is of a truly balanced design, there will be enough circuitry between the two different input jacks to help isolate the two preamps from each other.

The easiest way, albeit maybe not the best sonically, is to use some type of switch box between the two preamps and the amplifier. This electrically isolates the two, but at the same time, requires more cabling and introduces what would probably be quite a bit of junky low grade connectors, switches, etc.. into the signal path.

Short of manually connecting / disconnecting as needed, i'm not aware of any convenient yet good sounding approach. Sean
Thank you Sean,
Do you know is my 2nd-hand amp (Lexicon 212) a truely balanced amp. Also, any suggestion on a Good audio switch Box. thank you once again from a newbie.

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.
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
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.
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.
No use in this case because you want to send the processor through the tube pre instead of the other way around.
Thanks to all. I did ask Bryston regarding the input switch........... They said...... "The input switches are not made for that kind of repeated use and could damage your speakers if used in this manner."