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If changing R160 didn't have any effect, then are the +15 VDC and -15 VDC present at its two ends? Did adjusting both R160s not do anything on either channel? I suppose it is possible that something drastic has happened to both baluns (T1301 on both channels), if someone accidentally fed in too much voltage on both channels, but I would have thought that Q150A/Q150B would have died first.

A resistance of 56R between the centre of the BNC input and the non-earthy end of R130 looks to be correct - it is R10.

You might have dirty AC/GND/DC switches - these contacts are a bit prone to getting a bit mucky after years of service, but they should only be cleaned with isopropanol and carefully. It is easy to bend the gold-plated leaves if you are too aggressive and the board substrate can be damaged by other solvents. I have cleaned attenuator cam-switches, but not the AC/GND/DC switches. Others may give you advice on that. You may also have dirt on the attenuator switches, but time will tell.

You may well have faulty Q150s, but since they are dual FETs in the two channels, I would have though the failure of all four devices was a little unlikely.

I don't think that dirty attenuator switches would give the symptoms you describe, but I could be wrong, I often am. Dirt on these switch contacts usually has an effect on the frequency response of the affected attenuator, the dirt being capacitative. It's a pity that both channels are exhibiting the same fault, because otherwise you could swap components like the individual attenuator units and the Q150s between the two.

I have had a problem (still do, to some extent) with one of my 7A26s in that one channel was showing a different gain to the other. I think I got near to sorting that out by replacing the faulty U1350, but my memory is a bit hazy - not that that helps with your problem.

I am assuming that you have the appropriate Service Manual (you are quoting part-numbers) and that you have checked all of the voltage rails. That is often a cause of faults.

Good Luck, they are nice amplifiers,
Colin.