You could try changing the cathode bypass caps (C8 & C9). Remember you’ll have halve or double the value of the cap to hear an appreciable difference. You could also try lowering the plate voltage on the preamp tube to get a bit less headroom and add more grind. Also, you could easily add a solid-state full-bridge rectifier. It’s super easy to build using the 4 unused lugs on the rectifier tube socket and will cost about a dollar (4 diodes and some hookup wire). The SS rectifier will tighten the bass and punch up the mids (and it’s a little louder too). I put up a schematic for a switchable tube/SS rectifier version for the 2 Stroke here a long time ago.