Sonic Frontiers SFL-1 Sig preamp mods?

Hello All:
I have a SFL-1 Signature in my system for about the past 8 years, and overall very happy wtih the sound. Getting more bang for the buck than the Sig demands considerably more $. Before I sell it, is there anyone out there modding Sonics? I have tried a number of tubes, but wondering if anything can be done replacing caps or similar.

Thanks in advance,

Greg