the two things I would change are the input from single ended to balanced. simple mod, discussed elsewhere on head case, google is your friend.
and add the constant current loads to the circuit. You can find this in Audio Express July, and I forget what year.
there are also some grounding changes I would (did) make to get rid of some internal ground loops, yes it's our friends at Stax, they get the hard stuff right and the simple stuff wrong.
Also mwl168's advice is well stated
Cheers
Alan