If you've changed ANYTHING else, I'd look there first... If you haven't changed anything else, two possibilities exist: 1) You're imagining the change in sound, or 2) The performance of one of the components in your audio chain is deteriorating.
In the case of #1, I simply suggest that you imagine it sounds like it used to; and for #2 (that will be more difficult to track down), but if you use any tube gear maybe a tube or tubes are showing signs of aging/deterioration.
I highly doubt that what you described is the result of plugging and unplugging cables... Normally, a system sounds better after cables are unplugged and then plugged back in (at least in my experience).
In the case of #1, I simply suggest that you imagine it sounds like it used to; and for #2 (that will be more difficult to track down), but if you use any tube gear maybe a tube or tubes are showing signs of aging/deterioration.
I highly doubt that what you described is the result of plugging and unplugging cables... Normally, a system sounds better after cables are unplugged and then plugged back in (at least in my experience).