The improvements are in the upper midrange and high frequencies. The transformers now go out another octave than the average original transformer. Specifically, MK1 transformers varied from 25-55 KHz (-3db), the ones I wind range from 80-90 KHz.
I am proud to say they equal the revered Peerless 20-20 series and exceed them in half the insertion loss which results in tighter bass (damping) and more 10% output power. For the given power the core is smaller which results in lower iron loss. In every case I have found reducing core size to be a very good thing.