Due to a bad experience with what turned out to be wrongly-grounded (by the installer(s) AB boards, I'd given them a wide berth and said some pretty nasty things about Pavel's little board. I stand before you to-day to recant my earlier misgivings and to suggest that while they won't bring the world to Jesus or win the war for the Allies, they're a pretty good thing to have.
Had Holger at Erhard Audio not dragged me screaming and kicking into this revelation, I might well still be stamping my feet, looking in.
My three-odd months' experience with the AB, installed by Holger during his construction of my ST-120 (upgraded with Lundahl OPTs, which are definitely worth the extra bucks) informs me, firstly, that the things can live longer than 60 days and do no harm.
Which is damnation with faint praise, the perfection of which phrase I leave to PeterW.
The AB board definitely keeps its bias mud. Suspicious as I was, I had holger leave me bias probe points for each tube on the back of the amp, and a Fluke confirms this as rock-steady at 0.375/tube, which is rather an ideal bias capable of serving both KT-88 and El-34 tubes in their "happy range."
While I, as I expect many others, don't consider manual biasing a chore but rather a pleasurable duty, the precision the AB board maintains cannot be accomplished with the tiny bias pots on the driver board. Even the pressure of a wooden screwdriver can deflect the meter a needle's-width or more so ideal accuracy is not attainable this way.
I am not a fan of "post-blight" tubes and use some fairly expensive NOS bottles in the big holes. I do like knowing that they're not going to endure even a second's over-voltage on the grid, at least. Tube longevity is one of the Pavel board's selling points and while I haven't run it through a full 10,000-hour cycle, I'm certain that an upset bias won't be the cause of their ultimate deaths -- just old age.
There's no change in the sound that I can detect. I just consider the AB board as a cheap insurance policy and a more precise way of doing things.
A lot of misinformation about Pavel's board has been posted here. There are only two people one should direct questions to, and that's Holger and Pavel. Trust me, disregard anything anyone else, no matter how high up in the VTA hierarchy (except for Bob) says about them.
The after-the-fact installation of the board seems to be the most problematic, even with the PC board changes Pavel and Holger have made to them. So let me cut the baby in half: If you're yet to build one of the kits, put the AB board into it as you're building it. If you already own a fully-built and operating amp, don't bother with it. But DO check your bias regularly if you want your tubes, rectifiers, and power trannies to breathe a bit easier.