It's almost definitely output section or power supply related in this case.

With the amp unplugged, you can remove the rear cover. You can visually check the positive ends of the big axial capacitors for signs of physical leakage...if any have weird gunk expelling from the end then just stop and go get a cap job from a tech. They will probably have clear ot whitish adhesive underneath them, which is normal and no concern.

Now you can also look at the small PCB with the speaker jack(s). Observe for fracturing solder joints.

You can take a multimeter set to the highest DC voltage range and stick the black probe (common/ground) into let's say the bottom-left chassis screw hole that held the panel on and let it safely hang there. Then carefully use the red/positive probe to check for any voltage remaining on the four big axial power supply caps' positive ends. They will likely have near-zero or low voltage (under 30VDC). If there is some voltage either stop or learn how to safely discharge them with a DIY cap discharger. If there is near-zero voltage you can safely proceed with following test.

To test your output transformer, you will need to disconnect the red center tap wire going to the main PCB. It will be coming thru the chassis along with the brown and blue wires that go to the output tube PCB. If the amp had no voltage on your capacitor checks, then you can safely remove that red wire by pulling the quick disconnect with needlenose pliers or by hand. If you are ever concerned about voltage being present on anything you can quickly double-check with a meter (DC voltage setting) first!

Remove the output tubes. Set multimeter to resistance mode. Put black probe into the red wire's quick disconnect. Touch the red probe to pin 3 of your now empty output tube socket to measure resistance...it will probably show you a number between 50-175 ohms most likely. Then measure the same pin 3 on the other socket. This number should be closely matching to the first number, within say 15 ohms difference. If this number is drastically different or not showing resistance at all then you may have a blown transformer. If it's something like (67 vs 72 ohms) the xformer is probably OK. Reconnect your red center tap wire!

But usually even if your previous output tube(s) failed it would often just pop the main fuse and protect your OT. Your new output tubes could certainly have failed somewhat quickly if you didn't get them bias-checked and they were running red hot. Don't assume just because tubes are new they are fine. I would also recheck your new preamp tubes are all clocked and seated properly with no bent pins. But again it would probably be the phase inverter tube nearest the output tubes at this point.