You can do some math to figure out pass frequencies of all the coupling and bypass caps ad nausium. Trim it all to where frequencies useless to the speaker are squashed but 90% of what the speaker can use comes through. The speaker will still fart. The reason is that it's only in the range that the speaker can use anyway that the speaker farts. If the speaker encounters a frequency below it's range it simply doesn't try to reproduce it. That's the nature of it's range. Speaker farting is often best solved with a different speaker.
The amp itself farting in the PI and output stage is a common complaint for AB763 amps. Trimming the PI coupling caps is a good start. You can trim them lower than that and not lose any bass. Maybe .022uf. If your using something larger than .0022uf for the coupling cap feeding the PI you should trim it down to either .0022uf or the stock value which will be much lower.
I think the AB763 tone stack starts to act funny if you change the bypass cap for the stage feeding it. I usually prefer to reduce the value of the bypass cap the stack is feeding. Try a 1uf or a 2.2uf cap. The stack will still work like it did but there will be a 4 to 6db reduction in bass to the stage that feeds the PI. You probably won't hear a reduction in bottom end. Just a tightening of the bottom end. Any coupling caps over .022uf should be reduced to .022uf. Considering the impedances of the stages following the tone stack a .022uf cap is passing anything the speaker can reproduce. Higher is fartier (new word) only.
I often see the bias set too hot in these amps. A hot bias will break up sooner but also sacrifices definition. A cooler bias will have better dynamics. As long as you don't get disagreable amounts of crossover distortion you can cool the bias and get better bottom end much of the time.
Old power tubes have crappy bottom end.
NFB mods that reduce the amount of feedback loosten the bottom end.
Last but not least... The AB763 circuit overdrives the power tubes first. Followed closely by the PI and then the preamp stage feeding the PI. So... If your amp (not your speaker) is farting, you are simply dialing too much bass in the EQ for the amp to reproduce. Since the amp is accurate and clean up to the point where the power tube start to clip, if the amp is farting there is too much bass for the power tubes to reproduce. The "correct" method for getting maximum bass and no farting from an AB763 circuit is to dial up the amount of overdrive you want, set the mids and treble where they sound good and then set the bass control to zero. Now dial up the bass until it starts to fart and then back it down a hair to where it doesn't. The amp is now producing all the bass it can. Any more bass going through the circuit will only add flab, not more bass. It's not unusual with an AB763 amp that is heavily overdriven to see the bass pot set to two. Or even zero. Really. With a .047uf mid cap there are actually a lot of lows getting in even if you don't use the bass pot. Trust me. Most AB763 amps are farty because the preamp is capable of supplying more bass than the amp can reproduce when clipping. You MUST back the bass pot down or your just wasting watts on flab.