There will be lot of problem with the DC stability, if You apply 2 CCS instead of the resistors. You can use bootstrapping, or parallel CCS+resistor, if You need higher input impedance.
brute force would be electrolytics bypassing diode dyamic impedance - need 2x ~ 1000uF from signal in to each input Q base - Al electrolytics (or even Ta) won't be as bad as diode distortion
the split R, C bootstrap can give MOhm AC Z - although you may want to take off from pre/driver Q emitters to reduce load dependance - if your supply is OK I wouldn't bother with subregualtion
May you please comment the Elvee circuit (Post 5). Do you have any suggestion to improve it or I can just implement it as it is presented? What will be here approximately the input impedance?
For example is it possible to use a LED/2 LEDs to avoid the C1 & C2 in the Elvee schematic (Post #5)?
Or, are we shure the two CCS will not create problems if they conduct a (also slightly)different current?
so in the end: better 2x CCS with this up and down problem, or keep the 2 resistors as in the original schematic?