Thanks for the replies. I have read about the SB needing to be on for long periods of time. I have also read that if they come up to full volume in a few seconds as mine do they will not benefit from any more time switched on. I can't say I notice a big difference when run for a long time. I would like to try an non SB to see if they behave differently with it.
Reversing the channels is an interesting idea but unfortunately the problem seems even on both sides. Not a great surprise given vocals are usually mixed in mono.
I have never heard that you need to keep an amp on with a SB unit. Mine has performed for years without doing this. It only takes a few seconds for everything to engage. now, amps take a while to warm up, but like 5-10 min for solid state, longer for tubes. that might be whats up here. new info to me though either way.