I see it rarely and see quite a few come thru the shop, (the thread stated as "nearly impossible", meaning a few may if driven balls to the wall or an MFJ meter is used to view it) however in his case, I just think cascading amplifiers could be destructive if a good accurate power meter in not available. Sometimes if the right equipment is not available to look at critical levels, it could be chain reaction to destruction. Cable loss, VCC, saturation, current, input RF are all critical in cascading amplifiers.
The Motorola MRF-150 Data Sheet clearly specs the part at 50 VDC, 150 watts dissipation. Multiply that by "4" and subtract additional losses thru the combiner,LPF, coax loss, relay and whatever else is left will show less than 600 by far. In a perfect world with zero loss, I agree, it should do 600. I typically see with 70 watts drive the ALS-600 delivers a clean output of 400-450 watts with a pretty clean spread.
Now use a peak meter as he did in the link , whack the crap out of it on CW or SSB with a tone, you may get lucky and see 550 - 600 watts on peaks. Run it like that , get the box ready to send it off for repair.
Hmm... Phil drove his with less than 100 watts as did I. Doesn't seem aggressive. And of course a peak reading wattmeter is required. Also, of course, dissipation is not really relevant with SSB. You seem stuck in a loop.