Preamps with two main-outs

What I would do is disconnect the ic going to the amp/speakers not wishing to be heard, at the pre-amp end.

Interesting the stated effect...having to run both or nothing...

I run outputs from my sources (TT thru a phono eq, the 'puter, CD, tapes) into a matrix which goes directly either into the main input of an amp and/or to a receivers' tape in.  Sometimes an outboard eq gets 'looped in' via the matrix for input level control and/or eq, depending on the source.

This routine cares less whether both or either are running.. there's a slight drop in input level if both are running, but it's nominal....

Maybe I'm just lucky, or I'm not experiencing the 'feedback' that onhwy61 notes...any thoughts on how this works vs. the OP's state?

This is quite normal.

The input of the amplifiers are active components and need to be switched on to do their job. Obviously when one amp is switched off it is providing low resistance to ground (or frequency dependent resistance) and affecting the line level signal adversely.

The fact is that this is rather a dogs breakfast approach to hooking up audio gear. It may kind of work with some gear depending on the different ways the input circuitry behaves when switched off (due to different designs)

Honestly you need a Zone Mixer between the output and the two separate zone amplifiers as it appears currently you are just connecting the same output to two amps (dogs breakfast approach)

The dogs breakfast approach could also lead to ground loop issues and other interference/noise but on the positive side perhaps different interconnects may sound different in such a setup - providing endless entertainment going down that rabbit hole. (as I have stated many times - well designed gear that is correctly setup won’t need special cabling)

Good find!  I took a look at the manual for the SP-331, and it indicates that the amp is muted for some number of seconds after turn-on while the input tubes warm up.  That is not surprising, but what is very surprising is that based on your findings it would seem to be a good bet that the muting function apparently shorts the input to ground.  I have never before heard of that being done by any component. 

And if that is a correct interpretation of the results (as opposed, for example, to the SP-331 performing in that manner as a result of some kind of defect in the particular amp), I would have to say that it is a ridiculous design.  Consider the situation where it is the only amp being used in a system, and the user has a solid state preamp having a low output impedance and chooses to leave the preamp turned on all the time, while the amp is turned on only when it is being used.  And he or she chooses to play a CD through that preamp continuously for many hours, for burn-in purposes.  The preamp would then have the signal it is trying to put out shorted to ground, with the current drawn from its output stage limited only by its own output impedance and drive capability.  Damage in that situation being a very possible result.

Or consider the very situation you have, but with a preamp that has separate output stages to drive its its two sets of outputs (although relatively few preamps have that feature).  When listening via the MC2200 you would not know that the signal the preamp is attempting to output to the unpowered Vincent is being shorted to ground.  With damage once again being a possible result, eventually if not sooner.

On another note, I'm not sure how much seriousness there was in Onhwy61's comment.  But while it is possible that in  **some** cases having an unpowered component connected to the signal path might affect sonics, in a way that would not occur if that component were powered up, the reasons would have nothing to do with "out of phase backflow current."  I can expound on that further if anyone is interested.

Best regards,
-- Al