Which cap should I reduce next?

[Roe]

Do note that PI coupling caps are reactive components and if inside feedback loop such will effect phase of the feedback signal. If you dimension them too low (corner frequency high enough within audio band) you start to introduce positive feedback at low frequencies and risk amplifier stability. There is probably a decent balance of choosing low enough capacitance to minimize bias shifting and blocking distortion at sustained overdrive, and high enough capacitance to retain decent stability marging.

I would leave deliberate hi-pass voicings to caps that are outside the loop. They are more effective anyway because they don't have to fight against negative feedback.

No problem with the widely used 22nf caps really.

[bepone]

Thank you all for the input.

I notice that fender used a number of different values before the PI in the Brown era amps. The deluxe uses a .01uF and the bassman a 500pF (IIRC). Seems counterintuitive to me.

I think I will likely end up halving the values of the caps on both sides of the PI.

500pF - 1nF before the PI and low coupling caps after (100nF->22nF) after if cut is too much add step by step more capacitance. if you cut too much of bass then with some pedals sound will be too thin, shrill, so leave some acceptable amount

never compare any design with yours,this leads to confusion, threat every amp like unique plant.. play and listen what he needs and intervene in that way.. because main impact on the bass is coming from the output transformer, and yours and others OT is not likely to be the same. also important "filter" is the speaker box (1x12", open, closed, 4x12" all are different in the bass) ..

[brewdude]

A quick update…

First I tried .047uF caps after the PI—there was a moderate effect in bass reduction.

Next, I tried a few different values at the PI entrance. I settled on a .0022uF (smallest film cap I had on hand)—much more effective bass reduction, but had become a bit shrill.

So I put the .1uF’s back in the post PI location—this works very well for me.

Thank you to everyone for sharing your knowledge and advice.

[bepone]

you see, simple, enjoy

[brewdude]

Thanks.
I never thought of it as difficult. I was really more curious if there was any sort of consensus with regard to which way is more “right”, or likely to have a beneficial outcome.