>> Tube rolling
>> IRL510 PBF or N mosfets
>> Crosstalk mod
>> Change caps to bigger values and using better brands Nichi's or Elna's will do replacing those NCC's.
>> Change LM317 to 317A military spec fets
Everyone that responds to you will probably tell you something different, but this is what I consider the best
low cost mods that are easy and have very positive sonic effects:
1. Replace the caps with the highest audio grade caps you can buy/fit for the radial electrolytics and Wima or equivalent for the film caps. Know your size constraints; especially diameter. Height considerations can be ameliorated by using extra/longer posts. Increase the capacitance of the PS cap to ~10K and output caps to at least 1K. Put bypass caps on the cath resistors. If you don't notice a more pure sound from your amp, don't do anything else. Some folks can't and to keep modding is a waste of time and money for them. If you spend more than $40 on caps, you haven't gotten any deals or you made multiple orders.
2. Rolling tubes. You can obviously do this without swapping caps, but an incredible tube can only do so much alone. Further, if you don't know tube characteristics, brands, equivalents, fakes, etc, you can be greatly disappointed. Since your amp only has resistors to bias the tube and not trim pots (right?), its even more of an imperative to use well-balanced valves, since you can't adjust an individual channel.
3. Crosstalk mod. Are you actually bothered, peeved or pissed about crosstalk? If so, you might want to risk doing it. But do #1 first and see if you
need to make this mod. I never did. I don't know why, but I can't perceive it or it isn't an issue for me. YMMV
4. Changing mosfets. Meh for me.
Hope this helps... Take a look at my FrankenAmp, G2, on my profile, also.