Just solved it!!! Thanks so much to PB, Doc, Grainger, 2wo, TubeTim, vetmed for all the support along the way - and a special shout-out to the Queen for her incredibly gracious emails and encouragement in connection with fielding my replacement part requests.
The culprit (as explained in some detail below for those who are interested), turned out to be a simple bad joint on a resister which threw off the LM431's variable output, which meant inadequate plate voltage on the 12au7 on that side, which in turn meant inadequate voltage on +reg/OA/IB, which caused the LED's to stay dark. It is so immensely satisfying to have finally figured this out - made all the more so given the incredible sonics of the mainline. I'm listening to Pierre Fournier's recording of Bach's Cello Suites right now on my hd800's. I've listened to this album a hundred times on numerous systems, and I can't recall that it has ever sounded this good. The mainline's effortless transients, truly lifelike timbre and micro-detail as Fournier digs deep into the strings are absolutely amazing. Trebles are detailed and extended, but never strident, and the bass presentation is faultless - deep, impactful, and incredibly tight. I will always love my crack (which is a wonderfully enjoyable amp in its own right and will soon take up residence in my office) - but for anyone who has been sitting on the fence awaiting comparisons, be assured that the mainline absolutely takes it to the next level and is an absolute steal among TOTL amps.
I promise to post more impressions once I've had the time to do some real critical listening. For now, here's the concluding chapter in my trouble-shooting saga:
After discovering that my C4S boards fully lit up (and measured as mirror images of one another) if I removed the 12au7, I realized my issue had to be a regulator or b-socket problem. I started by measuring continuity on all the relevant b-socket pins (passed). Then, I spent a couple of interesting hours this afternoon fully labeling the schematics with tube pins nos., OA/IB/+reg, kreg, OB, Oa, Ba, etc...along with comparative voltage measurements at each of the foregoing.
In the course of annotating the schematics, I reviewed the datasheet for the LM431, and finally wrapped my head around the fact that the voltage on its output pin (Kreg) is a function of R3 and R4 on the regulator board. Lo and behold, touching up R3 (the 2.49k resister) was the cure to all my woes. In essence, if Kreg (i.e. the LM431 output - which is the 12au7 cathode voltage) is too low, the regulated HV going into OA/IB on your C4S will be too low to light up your B-side LED's! This was a heck of a learning experience. Frustrating at times, but all the more satisfying in the end - in fact, I may have literally been jumping for joy when I flipped the switch, saw all the LED's come up and then confirming that all my voltages were dead on. Thanks again to everyone who posted for your help getting there!
[Edit - to correct a material misstatement: Kreg is the LM431 output and 12au7 cathode (not plate) voltage.]
