I'd definitely move the jumpers to the front side of the board if that is what it takes to make you comfortable shortening the other wires (I prefer to avoid putting anything on the back of the board myself). Leaving the connections to the pots and sockets as long as they are is just asking for trouble. Leaving those leads so long adds up to several feet of extra wire to act as antennas inside the amp and it also leaves so much wire tangled up that you really can't do anything effective with regard to lead dress that would minimize their interaction. IME the problems you are having is spot on for a high frequency parasitic oscillation. The volume dropping out and pumping like a tremolo can be due to the oscillation causing your voltages to sag and this can then cause the parasitic feedback loop to drop in and out as the signal level drops below the threshold where it occurs. Also the fact that your NFB pot is effecting it, and that it is worse at higher treble tone control settings, all of which shows it is related to higher gain at higher frequency. I would guess that the issue is occurring somewhere in the wires running from the board to the tube sockets, but its hard to have any idea of where specifically. You could try to find it by chop sticking, but with it being a intermittent problem that only occurs with a lot of signal that probably isn't going to work. Does the problem get better or worse if you switch between split or common cathode for v1? I've heard people occasionally have some issues with the common cathode version.