Card A might have leaky capacitors, or at least at end of life. I have a Roland RAP-10 i got from eBay with this exact issue, super low and noisy output. I just replaced some capacitors around output area and it improved massively. Replaced them all and now it sounds just great.

Card B, might have a broken trace or shorted contacts somewhere. You can always give it a soap and water bath and scrub it well. Then just let it dry. Then look closely for any broken traces, or corrosion. You can also apply some flux around the chips and try to reflow the solder. I fixed a non responsive card like this, though it was not a Soundscape.

Card A, I wouldn't be so soon to blame capacitors. I'd be willing to suspect a broken solder joint somewhere on or around the many 8-pin, SOIC dual op-amps. I am not necessarily implicating the op-amps themselves: if a gain resistor has a cracked joint, that will definitely upset its gain level. What I'd do first before I break out the soldering iron though, is try cleaning the jacks with contact cleaner.
It is possible that a cap or two are bad or that an op-amp simply isn't doing its job, but I'd check for broken joints first.

Card B, I'd check for lifted/floating pins on the TTLs and the Sequoia chip. It's somewhat less likely but it's also possible that one of those LS244s by the ISA connector are bad.
It's extremely difficult to tell a pin is lifted by eye, you'll need a magnifying glass and probably an exacto knife.

Thank you so much for the info. I will definitely check out the pins and see what's going on there.