No, A3 and A8 wouldn't be 174V. That is pin 3 and pin 8 on the 9 pin socket. No pins on the 9 pin socket should read that high (although 1 and 6 will for about ten seconds when you first turn the amp on).
Yes, 3 is a ground. It looks like your amp is working properly now. I would poke around with a wooden chopstick and a cheap pair of headphones plugged into the amp to be sure nothing causes a popping noise when you poke around in there. If you have a loose connection and you poke it, you will get a pop through your headphones and that can help refocus your efforts.