It sounds like your power tubes are dying. Loss of volume is a sign that they are on their way out.
Are the Sovteks really 6L6s or are they the 5881s? The Sovtek 5881 isn't a real 6L6 and will put out less power in a circuit designed for a real 6L6, like your Peavey. The Sovtek 5881 needs a different primary impedance on the the primary of the OT and higher plate voltages to produce the same power as a real 6L6.
If they are real spec 6L6s, they are probably on their last legs. Modern production tubes don't last nearly as long as NOS types.
I'd definitely retube with a *real* set of 6L6s, 60W of tube power should bury any 80W ss amp without breaking a sweat. If fact, if you can swing it, you may want to get a set of NOS 6L6s, RCA, Sylvania, and GE are all great. If you can find them, the Sylvania STR-387 and RCA 7581A are excellent choices, but are hard to find and are expensive.
Also change the PI tube. If it is getting weak, it may not be driving your power tubes to full output.
Since the amp is almost 20 years old, I'd check the filter caps in the PS for good measure to make sure the PS can supply necessary current.
In fact, you may want to get the schematic from Peavey and go through it with a scope to see if there isn't some other weirdness going on.
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