I don't think you can add onto the older Spoiler or Distillate circuits, as they are board-mounted. I'm happy to be corrected.
*I'm going to back up and issue the caveat, it probably can be done, just not easily.
Just to clarify; your Spoiler 5... it's lower output is low compared to your Series bass? I would expect this. I've never had a Spoiler only a Persuader 5 variation of one, and it's output is just short of scorch. It is less than my Series bass though. I had an older Distillate that was comparatively anemic, but I simply ran the input gain on whatever preamp or amp I was using hotter.