I find this discussion increasingly of interest, particularly the exchanges between viridian and lewm. In the end the use of an SUT, like everything else is very subjective. Like lewm, I favor low ratios, 10:1 and 5:1 are typical. Please note viridian's preference for higher ratios. There is no right or wrong here, just difference. I too like Bob's Devices and use a SKY 10 often. I also use a Consolidated 10:1 quite a lot. Like lewm, I also try cartridges with and without an SUT. Most often I seem to like the sound with an SUT better, but not always. There is no generally accepted practice in my madness, but I have fun. For others seeking guidance, I believe this diversity is enlightening.
