I know that you said you replaced the quad cap - BUT - there is a possibility that one section of the quad cap has gone bad. See if you can beg, borrow, steal or buy a cap tester or a new multitester that measures capacitance. Now - WITH THE AMP OFF - measure the capacitance of each section of the quad cap. I know that your cap is an 80, 40, 30, 20 cap but - the capacitance from one section will leak over to the other sections. With a VTA driver board and an 80, 40, 30, 20 cap ...
The 40 section should measure 70 to 100 uF
The 80 section should measure 160 to 200 uF
The 20 section should also measure about 160 to 200 uF
The 30 section should measure about 40 to 50 uF
When you measure > the capacitance will take 10 to 15 seconds to stabilize as that cap section charges up from the battery in your meter. If the capacitance is either considerably lower than normal OR will not stabilize and the value keeps bouncing around - then that quad cap is bad.
Never assume that just because a tube OR a quad cap is new or near new that it "has to be good". One good test of a bad output tube is to power up (on an ST-70/ST-120) JUST ONE CHANNEL. If one channel won't trip the fuse and the other does trip the fuse then usually one of the two output tubes on the channel that trips the fuse is bad. You can also look at it this way - IF one channel will power up and play without tripping the fuse then - you have exonerated the rectifier and the quad cap as the cause of the problem. If EITHER channel (separately) will trip the fuse then probably the quad cap is bad.