Actually, there is a lab test on the 3808 here
, but it isn't "full" in that the pre-amp output which was your interest wasn't tested. It rarely is tested, possibly because a very low proportion of receiver buyers use the pre-outs, and those that do should almost never have good reason to be concerned about the output level.
Nevertheless, the test did show uniformly excellent results, including power(186 watts into 8 ohms and 238 into 4 ohms)which indicates audibly transparent amplification at all safe listening levels with the M80s.
It's good to hear, although hardly surprising, that providing adequate ventilation solved the problem with the 3808.