No surround Technics SA-AX530

Not a good sign. There should be a click a few seconds after you turn on the unit. That says the output section is OK. If there is no click, you need repair. Perhaps only some fuses inside, but it could be worse.

Also make sure that the speaker switch is in the right position, like A, not A+B. Also the headphone jack can silence the speaker.
Make sure that ALL switches are in the right position.

Hello, I searched all thru the service manual of the Technics SA-AX530 Receiver, and I have to say that there is not any way to turn the volume of the subwoofer up (or down) alone. The only control of the sound that affects the subw. level is the BASS control, but it controls also the bass at the speakers too.
Good luck.

That would depend entirely on your unidentified subwoofer and what IT needs. The Receiver puts out a usable Subwoofer feed on an RCA jack. That would suggest an RCA Cable, huh?

http://exaudio7.netserwer.pl/allegro/technics/sa-ax530_2/sa-ax530_7.jpg

check for shorts on the speaker terminals

An amplifier should never be turn on without speakers, it will overload internally.
make sure speakers work and try again.

look for a connector that reads "Optical"..if none, then not supported.

you blew your power amp board, you cannot hook them in series due to impediance matching and improper input levels, cheaper to replace reciever than the board

bad solder connections and or dirty controls.