I was in the same boat. I had both A21 and several JC1. The A21 has 5 fuses that affect sound quality (1 on back panel and 4 internal). I could never get the amp to sound good regardless of what fuse I tried because it deviated too much. You could try Audiomagic Super Fuse (which is almost a better version of stock).
A power cord upgrade is good. Other than that, you are better off switching to a different amp like others have suggested.
A power cord upgrade is good. Other than that, you are better off switching to a different amp like others have suggested.