It's not uncommon for amplifiers to become unstable if the gain is too low. The closed loop gain usually also sets the amount of feedback, and too much feedback can cause instability. I'd consider output at 1% THD too high. The amplifier may be starting to clip at that level. Typical THD for the amp, as I recall, is a couple tenths, so 1% is starting to overdrive it. I expect the 990 is good for at least 2 volts, so a volt should not be a problem.

National has made some interesting integrated amp devices over the years, but I've never fiddled with any of them. After I considered the cost to build one to the equivalent of a ready-to-run model, it didn't seem cost effective to me. Power supplies, heat sinks, and a professional-looking chassis always seemed to cost so much. I guess I could have salvaged the parts, but didn't get into that. Of course, I missed out on the fun of doing a home-brew project.

Good luck with the gainclone.

BIll