Doug, very interesting info on the output of Zyx. I happened noticed not by measure but by listening, Zyx Univ. (.24mv) is actually higher output than A90 (.27mv), since you mentioned a factor of 1.34, it makes sense.
I think to achive the best sound with Bent MU for Zyx 4ohm is to use 1:20 ratio, not 1:10. By design, S&B TX-103 on 1:10 is only good for cartridge with over 10ohm DC impedance not lower, info available on S&B website. If setup TX-103 on 1:10, no matter how hard to try on various loading, the result will always be less optimal. In the case of Denon 103, given it's relatively high DC impedance, only 1:5 ratio should be used on TX-103 which will defeat the purpose of a step-up.
I think to achive the best sound with Bent MU for Zyx 4ohm is to use 1:20 ratio, not 1:10. By design, S&B TX-103 on 1:10 is only good for cartridge with over 10ohm DC impedance not lower, info available on S&B website. If setup TX-103 on 1:10, no matter how hard to try on various loading, the result will always be less optimal. In the case of Denon 103, given it's relatively high DC impedance, only 1:5 ratio should be used on TX-103 which will defeat the purpose of a step-up.