Help! Transistors replacement AudioAnalyse A9 power amplifier

[navidmustafa]

Hello Experts, I have a question regarding the replacement of transistors in my AudioAnalyse A9 amplifier (Pure Class A, 50WPC). This amp delivers excellent sound quality, which I really enjoy (sweet, calm, with a wide soundstage). Unfortunately, two MJ15015 transistors have shorted, and after reviewing the schematic, my technician friend raised a few concerns and suggestions. I would appreciate your expert opinion on the following:

1. Why are the same transistors (MJ15015/MJ15016) used both in the output and driver stages?
2. Can we replace the MJ15015/MJ15016 transistors with MJ21193/MJ21194 across the board?
3. Considering the different HFE values (20 vs. 75) and Ft ratings (1MHz vs. 4MHz), would this be an upgrade or potentially cause issues?
4. If MJ21193/MJ21194 can be used in the output stage, should the driver stage transistors also be replaced?
5. Or should we stay with the original value transistors?

Your guidance would be greatly appreciated. Thank you!

(Schematic attached)