The caps in the DS5 might need to be replaced, depending on age, but my suspicion is that the symptom of that kind of cap death would be degradation of sound, not loss of sound. You'd notice more hum and noise. The fact that you get seemingly clean power to the bass from the DS5 indicates that the power circuitry in the DS5 is probably fine and that includes all the active aspects of the circuitry in the box. The signal routing is all passive.
Given that it's an intermittent problem, I would suspect the contacts in the switching jacks. You might try opening it up and looking to see what shape they are in. I'm not sure what jacks are contained your unit, but some contact cleaning spray, such as DeOxit might do the trick. At the very least you could exercise the jacks. I believe it's the Treble output that is the one with the switch, but I could be wrong.
Another possibility is a wire that was hanging on by a thread that has given up the ghost. Again, a perusal of the DS5 innards will reveal that kind of problem.
As far as the 1/4 cord goes, this might be a good incentive to either get a proper stereo to mono cable, a stereo to mono switch mod for the bass or to just mod the output to mono. I very rarely use the 1/4 output on my S I (and never on a gig), but I am very glad that it's in mono when I do.
