Switching preamps in and out

A completely different point:
Reads like the op wants to do quick repeated changes to compare the preamps.
BAD IDEA. Better to listen to one for a few days. Then the next and afterwards think if you liked one bettter than the others that way.
The part of you brain you want to have deciding is not the same one you use in short conscious attempts to 'listen'.
So chill out, and just listen to them for a few days each. (You could take notes if you must.)

I completely agree with Al, (as usual). My preamp, (Modwright LS100) and my amp, (Sanders ESL) BOTH MUST be turned off and the amp allowed to drain off.

If I try to plug or unplug the RCA cable from preamp to amp with either on, it makes a gut wrenching, heart stopping, horrible noise and will usually blow the fuses in the amp.

IMHO, always turn off any component(s) that you are changing cables to.

Turn the amp off before switching.
And I agree with Elizabeth on this one. You really need to take time to listen to each pre to fully understand what you are hearing. Too often the piece that sounds best in short term comparisons don't hold up as better over time. They just stand out during the test. At least that's my experience.
If you do make the hot swaps, please post the results.

Elizabeth got it right. I find it difficult to switch from albums to CD's (and vice versa) in one listening session. The first source always sounds better than the second one.

Thanks everyone for input. Here are components. Everything purchased here on Audiogon. I agree with long term listening but I want to enlist some other ears too, so we will be doing switching in and out on that night.

Amp is -VAC Phi 300.1a

Source is- Playback Designs MPS5

Preamps are:
-VAC Sig MKIIa
-Messenger
-Allnic L-3000

Cables are all single ended