As I said in the other thread, I would check continuity to the tube pins, specifically V1 and V2 since your PI and power tubes seem to be working since it works well at the power amp in.
Another test you can do is to plug a guitar into the input but with guitar volume turned down. Fiddle with the volume, gain, tone, and presence knobs and listen to hear if any of them change the idle noise coming out of the amp. This can help diagnose where in the signal chain the problem lies. For example, on the drive or more drive channels if you crank up the master volume and the noise to the speakers doesn't change at all that means the problem is after the master volume. If the noise increases with master volume the problem is probably before the master volume. This is a good excuse to go through the schematic and follow the signal path and see if you can localize the problem.
I can find schematics for the HRDv III Rev B, but that is 2010, so yours may be a Rev A. Rev B will likely be close enough, but if you can try to find the exact one that would be best.
I guess if you are planning on taking it to a tech why try to diagnose it yourself? Do you want to make sure he is honest about the repair?