May disappear...

https://images.reverb.com/image/uplo...eu7wltwetv.jpg

At any rate, if a 3133a, I'd be inclined to suspect the biamp switch (particularly if both MF and HF are 'out'). They're notorious for this.
I have to give my 4430 switch a jiggle now and then (similar switch arrangement).

Depending on where in SoCal you are, I might be able to get you started. Feel free to DM.