Switching preamps in and out

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

Mike, the output impedance of the MPS-5 is extremely low, as indicated in the third paragraph of JA's measurements in Stereophile, so I don't think that it would have any problem driving all three preamps at once, via splitters or y-adapters, even though the input impedance of the Allnic is only 10K. The input impedances of the other two preamps being much higher, 250K for the Messenger, and probably something like 67.5K for the VAC, which is what JA measured for the Mk II (non-A). The parallel combination of those three input impedances is 8.4K, which is hundreds of times higher than the output impedance of the MPS-5.

So IMO a good approach, assuming that the cost is worth it to you, would be to drive all of the preamps simultaneously, via splitters or y-adapters, and purchase the DB Systems switchbox to select which of their outputs is routed to the power amp.

FWIW, I used that switchbox extensively some years ago and I did not perceive it as having any sonic effects of its own when it was inserted into the system. Although my system then was not quite as resolving as it is now.

Regarding Wolfie's post, upon re-reading what preceded it I'm not sure that he was understanding that what I was talking about, given that the level controls on the amp would be turned down, was NOT the possibility of damage to the speakers (although I can envision ways in which even that might be possible). What I have been talking about is the possibility of either damage or degradation of long-term reliability of the input and/or output stages of the electronic components. If that wasn't clear, I hope he will re-read everything I said from that perspective.

Regards,
-- Al

I have only practical experience and my comment applies only to amps with gain knobs (like the OP's amp) where no...and I mean zero...signal gets anywhere in the amp when the gain is all the way down (rare in home audio, common in pro audio), thereby rendering the amp safe for changing input sources, which I've done thousands of times without issue (I am extremely lucky however)...wear and tear on the jacks are also a non issue unless you're pretty clumsy or really obsessed with constant switching out and in. Grounding issues will raise their ugly head even if you turned the amp off first, so there's that. Also, regarding listening for differences...it's possible to switch things around and hear differences immediately and decide if it's good or not so good...although living with things for subtle in depth consideration is always a good idea unless the auditioned item sucks too badly as life is short...all a matter of degree. Have fun in your frenetic input damaging preamp testing voltage mismatch explosion session!