It’s a complicated question, because the steelhead’s "Z" inputs uses autoformers (primary only) rather than a traditional SUT (primary + secondary windings). I don’t think most of us have a good understanding of autoformers in this application lol. I’m not even sure anyone else is using them here.
The description on Manley’s site is fairly good but still kinda hard to parse out. It sounds (perhaps) a bit more like a "current injection" head-amp in electrical function, versus a traditional SUT - where the "optimal" level Manley speaks of is going to be loading the cartridge pretty heavily (and that’s OK).
The "rough" gain range listed of 3 - 12dB is much lower than most head-amps and SUTs, but the extremely high "base" Steelhead gain (up to 65dB) covers for that.
If you get a SUT - it’s going to sound different than the autoformers, that’s for sure. Even SUTs can sound wildly different to each other! All you can do is try a SUT and decide which you like better. If you do, the SUT should be run at the lowest gain level (50dB) on the R input set to 47K and minimal capacitance load (0 is fine). Even 50dB is quite high, so you don’t want to push past the usually recommended SUT gain ratio for your cartridge. For an MSL Platinum at 0.5mV, I think a 10x SUT would be perfect here.
The "Sky" SUT Bob sells is excellent, and in your shoes - yeah I’d give it a shot against the Manley’s autoformers. The "Sky" has a warm, vibrant, rich sound with superb dynamics and big bass response. It’s not as refined or nimble as some of the other top tier SUTs (more expensive), but it’s a fun listen. Fantastic for rock. It really reminds me of a classic tube amp’s sonic style. And it absolutely clobbers the cheaper SUTs (entry level Lundahls, CineMag 3440A) IMO. My main complaint is the little metal boxes can be microphonic and they're also easily bullied around by stiff cables (both solve-able, but a PITA).