Most Marshall designs that I've seen have a 47pF 600V cap between the two anodes of the phase splitter. Did the same in my home made EL34 based 50W amp.

Interestingly, in a AC30 the VOX people use a 4n7 and a 250k pot in series across the other side of the two caps that go to the EL84 o/p stage grids (with 1k5 grid stopper resistors), and call that knob a 'Treble cut'...