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 subjected them to intense heating that could have "de-aged" those MLCCs.

-Class 2 MLCCs can exhibit bias voltage dependent 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.