How to connect a subwoofer to the MF M6i?

I run 2 RELs from my Musical Fidelity M3, it has 2 sets of terminals but I sometimes use only one the way Telescope described.

You would need to run one set of banana plug cables & one
set of spade cables off the same terminals.

How would I connect the sub to the speaker terminals though, if they are being used by the mains since there is only one set?

As Macdadtexas said, you can always connect to the speaker terminals of the amp if the sub has high-level inputs. The Musical Fidelity amp you linked to also appears to have pre-out jacks so you could use those to connect to line-level inputs of a sub if it has only that option.

any sub with hi level connects that go from the speaker terminals, they don't suck and of the power out of amp just the signal, to the high level terminals on the sub.

Rel, many Definitive Technologies, Vandersteens, many Velodynes.....

a lot of subs have these connections, just check their websites