The OPS 1 has one set of imputes that are 47kohms(wide open) that can be loaded for MM/MC carts with the two adjacent extra RCA jacks for different value resistance/capacitance loaded RCA plugs. If your model OPS also needs resistors for MC I'd order some now so you don't land up waiting for weeks. I'd start with 15kohms, 1kohm, 500ohm, 250ohm and 100ohm these should give you the ability to run most MC carts, if you think you want more precision at least your close enough to enjoy your vinyl while waiting for the new value.
I don't find my Joule system "pretty tubey" I find it more linear, extremely fast & quick, and very dynamic. I didn't realize how fantastic/incredibly they were until I installed the PC-1 and the W/P 8's which were capable of letting me hear the components abilities. I think you need a little warmth to go with a detailed cart with the Joule gear, I haven't heard the Merlins but I've been told you hear what you feed them.