Maybe 6 months old!  needless to say, I'm thrilled.

Agreed the stock tubes = meh. I had the crackle issue as well. I replaced and biased with Mesa EL84's. Pay attention to the color as when it comes time to replace the Mesa tubes you wont need to bias if you get the same color again. I tested the amp with several tubes GT, Sovtek, JJ (Tesla) EL844, and Mesa. Sovtek were okay but a little hard clipping and the bias variance was just in tolerance. The EL844 were hard to bias as they operate at a much lower level and they sounded like mush. The GT (Medium) and Mesa were of similar high quality and start to get warm at about 1 o'clock on the master. I went with the Mesa's due to the color coded bias feature. Biasing the combo requires a little disassembly and time. I would prefer to avoid that in the future. I replaced the the phase inverter with a Mesa SPAX7 (short plate 12AX7) as well. That does not require a bias to replace.

Seconded. I've can't remember the last time I read a post where every sentence made me shake my head.

That's true of any set of trace matched tubes by any company...Not just Mesa...Trace matching is nothing new...Mesa is using a very old and reliable approach for fixed bias amplifiers....Use fixed resistors...

The problem with skipping a bias check with these amps in thier stock form is that there is a bias pot...bias pots drift over time from gshock and degradation. Especially the crappy little blue pcb types...It's a good idea to check those...

If you really want to make your amp a true fixed bias, measure the value on the pot when it is adjuted proper and replace it with a fixed resistor...