As with any Peavey product, contact customer service at Peavey and ask for the schematics.
If the board is quiet with all the channel faders down, and then each channel adds a little more white noise - that is no one channel is a lot noisier than the others - then it is probably normal. Especially when considering the volume levels we are setting.
You can probably update the op amps in the input stages. A 5532 has a lot less noise than the 4558s, but it also draws twice the current. So a few 5532 in place of 4558 is OK, but a whole board fill of them might overtax the power supply. if you can find a premium op amp that doesn't draw excess current, go fo it. Newer generic types like 4580 might be quieter than the 4558s. Simple enough to try in one channel and see.
of course once you figure out what you have to go through to get at the innards, you may decide to live with it.