Upgrades:
Few modifications can be done. you can add (later, after evaluation period of generic design) J-FET transistors on input. Simply remove lower input transistors (those who have emitter tied to 47 Ohm resistor) and replace npn BC 550C with 2SK170, and pnp BC 560C with 2SJ74. This is the best upgrade, and you can combine it with following: replace 10K resistors tied to bases of upper input BC¨s with current source diodes (anything from 1-4 mA works fine, but measure voltage drop on each of BC's, must be four times 16-18 Volt.)
Output stage in class A: most common modification-everyone speaks about class A. Friend of mine even built and sold it commercially, with my approval, but without my commercial interest. I am proud on my electronic design, specially when I see his monster class A amps, with my artwork ticking in them.Class A always mean big-heavy-expensive. OK-here is what you need for full class A (this is for output stage only, rest of curcuitry already run in purest class A):
Double the output transistors-this means BD drivers connected to MJ as they are, but now four this Darlington pairs on each channel. Each output transistor must have his own 0.33Ohm/5Watt resistor connected to emitter. Adjust idle current at 0.63 A per each output transistor measuring a voltage drop on each 0.33 Ohm resistor to 208 mV turning 2K2 trimer (turn it slow  and then wait minute or two, keep an eye on meter). Increase output stage current in more then eight small steps, because bipolars tend to increase current as they go hot. Keep this in mind and re-check idle current every ten minutes in first hour, then every hour on first few days.
Double the heatsinks. Every output transistor must have at least 1C/W heathsink. If you decide to place heathsinks as sides of case, use two pieces, each for one side (channel) 400 mm long, 200 mm high, with 40 mm fins, and then place four output transistors  on "L" aluminum profile  40x40x4 or 50x50x5 with best insulators and lot of silicon grease.

Small but good modifications: you can apply them on generic or already upgraded design. First one is to replace 5.6 Volt Zener diodes with green 3 mm LED (remove zeners, then place LEDs with longer lead where marked end of zeners was). Then change 560 Ohm resistors to 180 Ohm value (four of them, two on each channel). Second one is to replace lower 10 K resistors connected between bases of upper input BC (cascode) transistors (those are two resistors connected to each other on one end). Remove  them and replace each with 15 Volt Zener diode, or use one 30 Volt zener connected between mentioned bases. you can combine this with current source diode modification, but now you need two current source diodes and one 30 V zener for each channel (instead of four curr. source diodes).

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