Subwoofer with Unison Unico

Is it possible to add a subwoofer to the Unico? It doesn't have a pre out.

You can choose a sub that has speaker-level inputs, or solder up your own voltage divider network with speaker leads for the input and RCA jacks for the output, or get one of these:

In either case, you essentially wire the subwoofer's amplifier in parallel with your speakers. There will be no effect on your amplifier or your speakers.

REL sub would work since it is designed to use a high level input from your speaker outputs.
I had a sub connected to my Unico at some point using speaker level connection.

I simply ran a second set of speaker cables from the same terminals used for the main speakers (in other words, from one set of terminals, i ran two different sets of speaker cables, one set going to the mains, the second set going to the sub)

No issues.