I take it those are the ohmmeter readings between pre-driver pins 2 & 9 - they look good.
To be honest, I can't think of further tests that I feel very comfortable about suggesting in the present context. Having said that, if you feel like carrying on with what might turn out to be tilting at windmills, you could try:
1. Measuring the DC on pins 1 and 3 of each pre-driver. These are the input pins - very sensitive, so a probe slip will almost surely result in the pre-driver being overdriven, though it should be awfully difficult to cause any damage. To make these measurements, it would be best to ground the meter's negative lead at a point that's close to the pre-drivers. You can probably spot the sort of Y-shaped circuit board trace that connects to both pre-drivers' pin 2 - that would be a good ground point for these measurements. The voltages on pins 1 & 3 should be quite small. (Tens of millivolts, maybe? I can't remember.) For each pre-driver, after measuring the pin 1 & 3 voltages, it would make sense to also measure pin 10 at the same time. That pin connects to the power amplifier output point - same as the TPs - and will have a much larger voltage, according to your previous work.
2. Force the protection relay to engage, and check the receiver's output meters to see if there's any indication that the power amp circuits are oscillating. You would do this with the receiver powered up (under a DBT rig, if at all possible), volume set to minimum, and the METER switch set to LEVEL. Now for the hard part. The least risky way I can think of to force the relay to engage is to identify transistor TR809, and make its collector terminal conduct to ground. The collector terminal is the middle one of the three terminals. You could use a small metal-bladed screwdriver or something similar to temporarily make a short circuit between TR809's collector and another of its terminals. (As it happens, shorting the collector to either of the other terminals will work for this experiment.) You should hear the protection relay click on when you make the short-circuit, and then you can check the receiver's meters. The meters should show zero output. They might twitch a bit when the relay switches on or off, but they should quickly settle down to zero and stay there. If the meters detect any output when the protection relay is engaged, then there is an oscillation issue, and amr2's precaution is probably the next step. (Especially because I can't think of any other steps in this case...)
Long-windedly,
chazix