I tack-soldered film caps a few of the MLCCs (also replaced two with film capacitors as I was trying some desoldering for off-board tests) and then removed the new low ESR electrolytic from the -15 V regulator output, tack-soldering a cheap 0.1 uf electrolytic cap in its place. Interestingly, the intermittent ripple didn't return, even after removing the film caps tacked to the MLCCs (the two film caps I fully installed remained). This gives some weight to the possibility that there were multiple failures (voltage/power stage output cap, supply voltage regulator cap, & op-amp decoupling caps) that caused the problem.

While researching capacitor types I learned a few interesting tidbits.

- MLCCS are generally available in two classes at present with Class 1 being the highest stability, highest cost, and least dense of MLCCs. While typically used for oscillators and not cost effective as supply caps, there can be additional reasons to use these (voltage coefficient of capacitance loss, piezo-accoustics, long term aging).

-Assuming the original (mid 1990s) MLCCs are Class 2,(possibly even Class 3), testing the repair options by tack-soldering film caps to the terminals of the old MLCCs may "de-aged" those MLCCs.

-Class 2 MLCCs can exhibit bias voltage dependant capacitance loss.

While Class 1 is roughly 4x the price, I'm proceeding with them instead of film caps or the cheaper Class 2 caps on CH1 to give this effort to minimize the remaining ripple its best chance of success.