Having directly compared the two, the Lundahl LL1931 is considerably better than the S&B TX-103/Bent Audio Mu SUTs. They do, however, require several hundred hours of break-in to sound their best. Merely my opinion (and my system) and the usual disclaimers apply.
What Step-up transformers are best value?
I have recently bought a VDH Black Beauty cartridge. It needs top-flight SUTs. I'm currently using Silks, which are very good. I have also tried the S&B TX103s (lent by a good friend), and they are noticeably better. Their price, however, is prohibitive (£1400). Anyone know of any serious bargains out there, which are up there with the Tx103s?