I know I am probably a majority of one on these boards preferring SET amps with Stax transformers over Stax direct amps, however, once in a while somebody asks about transformer upgrades for the standard Stax boxes.
I was going to get a Woo Wee. However, having heard Woo products, I was very uncertain of the transformer quality.
Instead, I tried a pair of the new manufacturer Z-565 Dyna OP's from Triode Electronics wired in reverse. I removed the standard stax transformers, and appended the Z-565 trannies to a breadboard for wiring. The Z-565 run about $210 delivered, less than the lundahl and James types often cited.
I have to admit, I was always pleased with the standard stax boxes, but these trannies are a significant, not minor, upgrades. High frequencies are much finer and better defined, soundstage about 20 percent larger, and bass much better defined with lots of internal detail. I have been using it with my fave 300b.
For DIY types who also like transformers, it is worth considering. It will probably also require a plexiglass shield to cover the bias circuit, which is exposed in this breadboard.
So far I have only been able to try it with standard low bias Lambdas, because the pro socket on my Spritzer mod SRD 7 box has not been working, but plan to get the 507's in action as soon as possible if I can get the pro socket working.