I really like the Benz Wood SM for the money. Going up the Benz line, however, can be tricky. When they swap over to ruby plates (Ruby, Gullwing, LPS), the electrical characteristic changes a lot (much high coil impedance for each output level). I’m a SUT guy so I don’t really dig the ruby LPS (doesn’t match well to SUT)...or maybe it’s also that I just prefer the warm, lush, robust "color" of the wood bodied iron-cross models. Your Ref 2SE is about ideal for the ruby models (I had a Ref 3SE), but as you noticed, noise will creep in as you go below 0.4mV. So you could try the high output ruby or LPS, but they’re carrying an AWFUL lot of coil at that point. As for the higher Benz iron-cross models - I find Zebrawood L and Ebony L to be a bit more refined than Wood SM, but perhaps not significantly so. I think you’d be looking for a bigger change than that.
I think Koetsu is a good choice if you have arm to support it. I’m not sure about the Rega arm - haven’t tried that combo. On paper, it’s too light. But on a good arm match, the Koetsus have even more of that warmth, body, and vivid midrange which I like the Benz Wood for. With more refinement to boot. The potential downside is Koetsu’s treble is more relaxed then Benz, so if the rest of your system is dark that may be too much. Koetsu’s output ratings are good, so the 0.4mV model won’t give you trouble like the SS did. Even the 0.3mV Koetsu models (Platinum magnets) have enough output to work very well with ARC Ref.
Don’t do an Ortofon, you will miss the sweetness in midrange Benz and Koestu give you (based on A90 and Windfeld Ti). The Wood SM is already better than Cadenza Bronze so you’d have to look above that, which is where they get sterile in the midrange (for me). I haven’t tried Lyra, but they have a rep of being on the brighter side.
It might be a "none of the above" scenario, as @mijostyn suggested. I've heard only good things about the MSL cartridges, so far.