Unless someone 'messed with it', I cannot see how the 'tubes' can just go bad. If anything pull your cables and clean both the cable ends and the chassis connections. Turn the system on and let it run for a few days. Also did you by chance unhook the speakers and get one or both out of phase? Anyway, just some things to look at.
Do I have tired tubes?
During the summer when I went on vacation I unplugged my Sonic Frontiers Line 1 pre, my Musical Fidelity A3 CD, and Anthem MCA 20 amp. The pre I bought used and was around 2 years old when I bought it just over a year ago. It has the stock 6922 tubes in it. When I got back home and plugged everything back in by bass was a bit muddied and the midbass was a few dB louder than normal. The resut is now boomy poorly defined bass. Does anyone think that the tubes are the culprit? Perhaps they are due for replacement?