Stellavox amplification help please

Georges Quellet developed the Stellavox line of tape recorders 70 years ago. in the early 1970's he introduced his SP series which lasted for 20 years until the company went defunct in early 1990. That series used the same potted/plug-in amplification module originally designated SPA-SOA for most of the the whole time period. The later modules were un-potted and designated as SPC-SOC. The same circuit was used in their acclaimed TD-9. View attachment img20200215_09044655.pdf

The schematic of the module appears below. The "P" refers to the two transistor preamp and the "O" refers to the four transistor output amp. Four of these were used in each tape recorder; two per channel; two per each playback and record function.

I'm looking for any suggestions for updates to the circuits - not total rebuilds. A few folks have contacted me with improvement suggestions including adding constant -current diodes but I never followed up on them. Your input is appreciated - and thank you.