Counterpoint 5.1- Beyond My Comprehension

The 6CA4 inthe SA5.1 is the rectifier (note: not regulator) for the high voltage B+ of the SA5.1.
It converts AC to some 500V of DC that goes into the regulator.
If the B+ going into the regulator has low voltage, then obviously gain will be diminished.
Glad you didnt damage the regulator by replacing the 6CA4 quick.

I have done upgrades to SA1000, 2000, 3000, and limited stuff on the 5000 which is very hard to work on.
The SA5.1 schematic is floating around the Internet and the official one available from Catherine Weber per Altavista. Both Bigkidz and I have similar ideas of what Mike used to do with the mods (chokes, resistors, oil bypass, naked vishays, Non inductive WW plate resistors, etc.) It wont be 100% what the premium / gold level mods Mike used to offer but the ideas would be close.
Bringing a box to spec by replacing noisy resistors and old lytics is easy. Executing the mods take a lot of creative mounting work due to the 1U size of the SA5.1 vs the the HUGE size of the oil caps and the modern better sounding coupling caps (there is a reason why Mike sticks with Dynamicaps)

-Johnson

The 5.1 is good as it is as long as it's stable. I wouldnt sink money into it unless there is some sentimental value.
There's really nothing special to a dual 6922 grounded cathode parallel plate-drive line stage. An SA5000 with the followers pulled is essentially an SA5.1.
The Phono stage is a 2 stage 6922 with grounded cathode. Really nothing special.
The Power supply... The section after the 6CA4 rectifier could use a choke. The tube regulator section is in the main chassis I recall and without a service manual I wouldnt start working on it.
you can just do a signal path mod by changing out resistors and coupling caps (the output cap will be smaller, less bass, after the mod as I recall it uses a 10uF RelCap. Modern high end caps are 10x the size)

Bigkidz the 5.1 is pure tube rectifier and regulator contrary to whats written on AltaVista. The 3.1 is solid state.
I think even the voltage reference uses a gas tube.

With all the counterfeit 6922s floating around your money is better spent on caps and resistors.