Well, I put some JJs ECC83s in my TB500. The EQ is definitely more responsive. Overall tone? Not sure if it's changed as my hearing sucks. The original tubes gave only a cut effect for the EQ knobs, after 12:00, no change. Now I can boost the mids to 3:00 for a noticeable increase... Yes, call me crazy, I don't do the "smiley face EQ" with my bass.. I actually prefer some mids to cut though. Anyways, the JJs were well worth the $28 I spend for the pair.