I'm using the clock on the Metron itself. It does have a clock in though, so I could possibly use a Polyend Poly 2 to send clock from an external drum machine to the Metron as I don't think the Digitakt or the Octatrack has a clock out port, only midi.
I've been eyeing the Metropolix @troux. It's a tremendously powerful pitch sequencer, and the two voice control is exactly what I'm going for. The issue I keep coming back to: I fail to see how it could interplay with the Metron. I absolutely adore the Metron's 'variations' capability; I love being able to flip to a new musical idea, and then flip back to the original before moving on to something else. With the Metropolix I kind of lose that ability, though it might be true that with this recent change to precision adders generating the pitch sequences I may have already lost that ability.
Though it pains me, maybe I should jump ship from the Metron sequencing nearly everything to an external drum machine sequencing drums while the Metropolix sequences pitch? I love my Metron, but it's definitely proving difficult to integrate certain aspects (in a tasty way).