Ya know, I put in a pretty decent TKD pot in my Crack amp. I've used them in a couple of projects and I've found to be a fairly good performer. They are kind of expensive for what you get (around $70) but to get a much better pot you have to spend a lot more. I've also tried some of the "made in china" DACT copies but I don't really like the sound of those SMD resistors. I think you'll find a pretty small gain in performance for the money spent on fancy pots. To me, it's kind of the icing on the cake.
After spending some time with the Speedball upgrade, I have no problem sticking my neck out and saying that there isn't going to be anything you can replace in this amp that's going to equal what this does.
The sound is more detailed, , the noise level is reduced (which may be revealing the improved details), the bass seems to go lower and is tighter, and there's a few dB more gain. Without doing any A/B testing with my stock Crack amp, I found that all vocals sounded more true to life and instrumental separation is better. Best of all, the amp remains very musical and doesn't become clinical which can happen with many well intentioned upgrades.
It's still just as hard to rip yourself away from the music. That's always the "acid test" for me. When I change something then listen to some music to see how the change effected the sound, and all of the sudden I realize that I've listened to a whole album, I know I'm on the right track.
Bottom line is, the improvements aren't subtle.