In my opinion if you have intentions to overdrive the amp don't go too low with the coupling cap .
Going to adjust C3 + R9 as well as C4 +R8 to adjust low freq cutoff point. How should I change these values to lower my cutoff point? Lower them right? Any idea where to start for bass?
to know you have to use the famous formula of the frequency corner (F=1/2piRC) , there're calculators on websites that you can find and use (AMZ muzique). You'll see that putting big cap isn't necessary . I can say you the .1uf is unnecessary big here , 0.022uf is large enough. (works the same for the caps with the cathode resistors)
You can also try to reduce the Anode voltage of the EL84 , seems pretty high (that can be calculated too )
Keep in mind also that low frequencies require more power from the power supply in general. But that can color the sound and the behaviour of your amp (sag ) .
Keep in mind ,also , that guitar Output transformer , espcially mass produced ones , can be limited in bandwidth so they may not give good results with bass instruments. But once again , that can add colors to the sound .
the feedback can be a good thing if you want to keep the amp really clean , but it needs some knowledges on how doing it , that's not that simple. you can also make it variable , or even frequency variable but that will become really complex for such a small amp
For suggestions, I'd make the anode resitor larger as in bass amp (47K) to stay really clean (we can imagine putting a switch to select two kind of biasing). I wouldn't put big caps to keep clear sound , I'd soften the sound, find a simple and good tone knob and finally make the EL84 working good.
I'd be interested if you can say some words once your modifications done about the EL84 with bass.
