It should be no problem at all running it from a 50Hz mains power since the PX-8 takes DC power anyway
Is the power supply you are using providing 9VDC?
Is the signal diminished even when all loops are bypassed?
If so you can test where the signal is being interrupted by using the Insert Send and Return jacks to isolate Loops 1-6, or Loops 7-8.
1. Guitar to PX-8 Input, Insert Send to amp. Test for clean signal.
2. Guitar to Insert Return, Output to amp. Test for clean signal.
Also, internally there are a pair of jumpers that allow you to bypass the output buffer completely when moved over to the next set of pins (there is a diagram shown on the circuit board showing the positions for True Bypass or Quiet Switching). If the buffer is what has had a fault, setting these to the True Bypass position should at lest get you down the road until you can get a chance to send it in for repair.
If you are still having troubles after trying the above tests, we would be perfectly willing to take a look at this unit as a warranty inspection/repair.
I hope that helps!