My "work:" I would put the owner's PAS preamp on the bench along with his power amplifier, and remove all the load resistors. I substituted a ten-turn variable resistor in place of the load resistors, while sweeping the line input through the low-to-midrange, and watching the synchronized output on a scope as I adjusted the resistor. I'd do this at several different output voltages but generally that didn't make much difference as long as I kept to "reasonable" levels. When satisfied that the response was as flat as I could get it (generally I weighted my judgment more heavily to 40Hz and above, while making sure the 20-40 Hz range didn't go too far off), I'd measure the potentiometer and use 1% resistors to make up that value load resistor and install it. Same was done on both channels. The specific tubes used in each channel were noted and the owner advised that if he rolled tubes, he might change the response. Of course the owner would also have to use only that particular power amp or another identical one.
I generally found that in a stock PAS, the lowest I could go was around a 25K load. Below that, irregularities appeared, not all of them in the bass range.