In hooking up stereo and multichannel systems, does it matter if
different preamps drive the same amplifiers, if only one preamp is
turned on at any one time?  That is, do "dead" unpowered preamp
outputs interfere in any way with other, live ootputs that are twinned
into the same amps?  In real life, I have a multichannel preamp, and
also a synthesized surround Yamaha DSP, and they both share the same
amps.  Only either the multichannel preamp or the DSP is powered up
any any one time.  Their outputs are twinned into the appropriate
amps.  The system sounds excellent.  Is there any hidden or
theoretical problem here?

Couple of potential pitfalls. If a preamp is turned off but still
connected into a power amp, the output of the preamp could float and
induce horrid hum. You might also get hiss due to the input being
un-terminated. Some badly designed power amps are also prone to high
frequency oscillation (above audio) when fed with ?a bit of wire?. All
that is heard is a hiss but the output gets very hot very fast. But if
it?s sounding fine and not hissing or humming, no worries.

Best