He used to design medical electronics, and has designed a lot of PCB's that were rather critical, so he knows electronics, and how to do it right. He has a section on
his website about why he uses them, but as he puts it, PCB's that are well designed are good. Turret boards that are well designed are good. Bad design on either one is bad.
He uses the turrets on his PCB's because they make it easier to modify them. Makes sense to me. And yes, he runs his traces on the back, and uses the top as a shielding plane. It probably doesn't make a big difference - particularly in something as noisy as a tube amp - but it can't hurt anything either, and if you are going use a circuit board, well, why not? What he DOESN'T do is mount tube sockets, jacks, pots, or anything like that to the PCB. Basically, he is just lacing all his turrets together with the circuit board.
Personally, I like making circuit boards, so while mine are nowhere near as good as his (no plated through holes, single sided, etc.), I've still done something like that on my amps. I like what he says about the turret/eyelet board vs. PCB debate though. When amp builders are starting out and their production is low, they'll talk about how turret boards "sound better," or are "more reliable." But as soon as they get popular and need to increase their production, it becomes, "PCB's are just as good as turret boards." Which of course is true, as long as you design them right.
Gabriel