That seems about right. I don't know what else you can do without a scope. It could be that you damaged them when you soldered them to the MEHSA insulator and that what caused their failure. To find virtually any other problem, you'd need a scope to check the drive waveforms.
Connect only remote and ground (no B+). Measure the DC voltage 'directly' on the leg (at the body of the transistor) of EVERY power supply FET. If they ALL read the same, post the voltage. If one or more are not the same, post the voltage on the one that's different and the the voltage on the others.
It's VERY rare that any of the power supply FETs survive when any one fails in a power supply. If 4 needed to be replaced, you should have replaced the other 4 also. I'm assuming that the other power supply was not damaged.
Listen to Perry on tis amp he knows theese amps well as he helped me fix a few of them.
The noise your probably hearing is loose windings on the transformer Every 1500 bd Ive fixed had loose windings on the transformer so not that much of a deal try twisting/pulling on the transformers to see if the noise goes away or gets worse.
Also like perry said if you only replaced 4 fets that may be part of the problem.
All the Bd's ive dealt with a few fets were shorted and the other ones were leaking between the legs In the same bank of fets. There are 2 banks of fets in this amp 1 for the first supply and 1 for the second.
When i replace the power supply fets I also replace all the drivers for the fets as well the MPSA56 and MPSA06's i also check the gate resistors make sure they are within tolorance.