Right now I have quite a few transformers in the chain, and apart from one of them I am unable to find them sounding bad.
I have tried it with every source material, doing A/B comparisons level matched and blind to make sure I'm not fooling myself. What is the weirdest thing is that I put my Knif Pure Mu (Lundahl I/O) in front my NTP 179-120 (Jensen input and Triad output) temporarily, just to have some sort of level control before the fixed-threshold NTP compressor, and it just works wonderfully.
There's a Sowter step-down transformer at the input of my custom passive EQ, and that is the only place I'm considering trying out a solid-state input. It doesn't sound bad, but it just doesn't work with everything like the rest of the chain does.