The singles aren't hard to match (for Idss), and most of the time are closer matches than what is in the duals (at least from my experience). Linear Systems makes the LSJ74, which seem to only be available thru diyaudio.com in lots of 8 for ~$50. I think the 2SJ74s can still be had? There had also been a run of heat-sink thingies for grouping pairs of these into a single connected unit, but those are probably unobtanium anymore.
I would pull the 2sj109 and test it if I were you. Someone familiar with the SRM-3 may pop in and give you more enlightened advice.