JCM-900 4100 Super quiet both channels

[Mxzx]

Amp is really quiet on both channels, you can turn it all the way up and not blowing you out.

I switched the high/low power switch back and forth and the amp has some really loud pops, like the power section is working properly.

I was using the effects loop, is it possible the amp is "stuck" in some mode from the loop being used?

Could tubes be causing this issue?

Thanks

[Mxzx]

Well it appears to be the Impedance Selector. Switched it back and forth a few times on standby then turned on and is loud as normal.