Greetings immatthewj. If you’re only seeing one diode rectifier, it’s probably the negative bias supply rectifier. I do put HexFreds in the bias supplies of my personal amplifiers. Had a hard time telling if it really helped there though, but figured it couldn’t hurt. Because like the old saying goes, everything matters. I do know adding more filtering to the bias supply definitely makes a cleaner sound quality.
The V12 amps should have four strings of three diodes totaling 12 diodes for the high voltage supply. There is such a high current draw (large caps) that there are three in series for what should be one diode in order to keep them from blowing out from the turn on surge. I remember doing a few amplifiers (not just V12’s) with HexFred’s, but that gets very expensive as you’re installing 12 so most people didn’t want to do it. In fact, I think on a pair of CAD-805 amps we installed 16, four sets of 4 just to make sure the amp never came back with an issue. Now days, there are higher current HexFred’s available that can be used but you still want a couple in series. I’ve had to buy some of these because I use so many large value capacitors in my mono-block PS’s. My amps have two amp chassis’s and two power supply chassis’s.
It’s not that you’re drawing a lot of current tube wise. It’s the turn-on surge to charge the capacitors. The more capacitors, the bigger, the surge, and the more the rectifiers see a short initially at turn on.