First off, it would be helpful to know if the opamps are new builds and/or if they have been tested in something else. Starting up with 3 freshly built opamps in a freshly built FC526 is a definite no-no! So please, anyone posting for help, let us know those details first.
Having an issue here, when the comp is engaged getting a very thin sound, almost like a HPF as been rolled up to 400hz. Could this be in the Q-bias setup? I've always turned my trim pots CCW before I adjust, did you have this preset or anything?
None of the trim pots should be turned before it is specified in the calibration doc. They are all in their factory set position which is somewhere near 50%. Turning them all CCW is for sure not a good way to start especially if you look at step 14 of the calibration doc which states that the Q-Bias trimmer should be turned
CW. I also have it bold and underlined in the doc.

I would recommend to turn GR off, monitor the output level, turn the Q-Bias trimmer CW until the level stops increasing. This will take the VVR FET out of its conductive range. At this point, measure the freq response to see if it is high passed still. If so, you have a problem with the audio path somewhere. We'll go down that path if and when we need to.
PSA, when adjusting the Q-Bias trimmer, turning it CW will let the signal level increase. Turning it CCW will essentially turn the signal level down. Kinda reacts like turning a volume knob, in a logical fashion. Hope that makes sense. It's opposite of an 1176 so keep that in mind, or just follow my calibration doc to the letter and you'll be fine.