I recently finished a Bassman/JTM45 Micro build based on @robrob 's circuit design. I ordered a 12AU7 (ECC82) with all my other tubes for the build but also found a $20 12AU7 in stock at a local guitar shop. I can't really tell any difference in person between the two power tubes. I thought it'd be interesting to see if I could tell with any noticeable difference recording and analyzing the EQ levels in Logic Pro. TLDR, the difference is only within 0.2 - 0.4 dB (negligible).