The A-400 is the real deal. Bottom line is that as good as the 753's are however and I am sorry to this they and I strongly believe this will also be the case with the 555ESD are still not up to getting anywhere the best from the A-400.
Thanks - another factor which I have not mentioned is that I need a phono stage - universally it seems the internet suggests the A-400s is not great.
Also I have found really interesting info on the 75x crossovers/internals especially the tweeter resistor value being too low, randomly changed over production lifetime, mismatched on a production run, and even missing. So I'll be taking the 753s and 752s apart and seeing what is in there. The info seems to be from a reliable source who clearly understands electronics, and although they suggest that the tweeter was never the greatest, having the correct resistor removes the imbalance.
To my ears, the 555ESD sounds amazing directly into the MW60 headphones - more importantly, it has probably the best overbuilt frame wrapped around probably the best mechanism, filled with OTT components - if you are interested in CDs as a source, do you know about the Lampizator website/research? If so please have a delve through this, starting with the info on the 555 of course.