can a RCA switch box be made for 2 pre amps?

Of course, the better route would be to route the L/R outputs of the processor through a HT bypass on the tube preamp but, if that is impossible, a simple input switch would work.

2 pairs of RCA inputs, one pair of RCA outputs and a 2pole/2throw switch (switches the HOT from each while leaving all the grounds connected). I would suggest a shorting switch if you ever want to use it with the power amp on.

You might even consider a 4pole/2throw shorting switch (switches HOTs and grounds) to completely isolate the two preamps.

Kal

Radio Shack and Parts Express sell A/V switchboxes for under $30, but my experience with the Radio Shack unit is that it is unacceptable for both quality audio and video. Ditto the Switchcraft rotary unit. You can build your own that would be much better. Or you can use the PS Audio preamps as switchboxes (do you still have the 4.5? or 5.0?). Plug your other two preamps into separate inputs on the PS Audio and connect the PS Audio tape output to your amplifier, then use the tape out/rec out switch on the front of the PS Audio to switch between the Sonic Frontiers and the Rotel.