I can see where this is going...bass mid treb seems to work for most guitarists...it will fit in a 1590B, if not like this,
then with all knobs on top. One and 2 knob eq's are a compromise IMO, so is a 3 knob, but much less so. The Marshall guvnor tone stack works well too (on a guvnor). The single tone and trimpot "presence" control on a crunch box work well, would like to give that a go this wkd too. I've found that with a 3 knob tone control, most of the parts can be mounted on the pots themselves and the rest air-wired if necessary, don't really need a PCB.
The Q5 is a buffer so it should drive the tone controls no worries, but a buffer or gain stage on the end might be needed, which you did mention Rob.
I'll see what I can build this wkd, speculating gets me nowhere.