I think you'll get a lot out of bypassing the audio output caps with teflon caps. I bought a pair, shipped from I think Lithuania, for about $10. That was a great upgrade. It was the best bang for the buck mod I've done.
I also use Tung Sol 5998 and Mullard 12au7 nos tubes. The pair cost $150, and sound very nice. However, here's a $35 nos solution- GE 6as7ga output tube and an RCA 12au7 clear top.
I also bypassed the last cap in the power supply. Not sure that made much difference.
I've not bothered with the volume pot. Seems the greatest issue with the stock pot is low level linearity. However, as I've padded down the volume pot, I no longer use the volume pot typically at its lowest range.
I also changed the input RCAs to cardas. I really don't think that made much or any difference but they seem to be finished a little nicer than stock rca jacks.
Finally I'm using a diy power cord- Belden 19364 which is 14 awg (I think) and has a shield and a drain wire in addition to ground, neutral and live conductors. I used a right angle iec which is nice since the power jack on the Crack is on the top plate. The male plug is Wattgate.
Crack/Speedball, SEX 2.1/C4S, Bryston BHA-1
Sennheiser HD600/Cardas cable, Beyerdynamic T1 2nd Gen, Hifimam HE560/Hifiman balanced cable