Yes, I think the Cube Audio sub is the next step, although I'm thinking of the 10" since my room is slightly smaller than yours. Right now I'm using a KEF KC62--a mini sub that does a credible job.
I prefer both the Enleum and the Zen Triode without a pre.
I'm not tempted to try mono block Zen Triodes since 1) I don't feel the single Zen needs more head room--although the monos might offer other benefits as well, just twice as many tubes to maintain 2) I think the Enleum may end up in the system the lion's share of the time. It plays better at lower volumes than the Zen, although that might have more to do with controlling volume through the TotalDAC instead of the Zen dual volume controls. I'll have to experiment.
I sent the iFi Pro iDSD back--a great a value but fell short of the TotalDAC D-1 Dual.