Per the specs of the Manley Steelhead, the maximum rated input voltage is 50 mV. Your MSL cartridge with a 0.5 mV output is stepped up to 10 mV with the 1:20 MSL SUT. Maximum cartridge output voltage can be as high as ten-times the cartridge’s rated output when playing LPs. Therefore, your MM phono stage should have an overload capability of at least 100-mV for the 0.5-mV cartridge. I looked at the specs for an Audio Research Ref 2SE, the maximum rated input voltage is 250 mV and I used higher turns ratio (1:26) SUT’s with 0.4 mV output cartridges and never had any issues. I think most phonostages have a higher max input voltage capability than 50 mV. Also, I didn’t see a single Steelhead user in this thread indicate that they use(d) an SUT. I would poll the Steelhead users to see if they use SUT’s, and if so, the turns ratio and output voltage of the cartridge. This also seems like an appropriate question for Manley. In the meantime if you can get hold of a lower rated MC < 0.24 mV, and see if the issue persists.
I’m by no means an expert on this subject, but all this is based on what I’ve read from a Steelhead owner that was using a Cotter Mk2L (1:70 ratio) with a 0.28 mV output cartridge and was having similar issues as well as my own experience with SUT’s.