The STK modules are just the output Darlington pairs and associated emitter resistors and a few other bits, all of which are likely to be fried when an output shorts. I've never noticed a particular difference in sound quality, every amp I've seen with STKs has good specs, and I was quite happy with my KA-80 when it worked, and now that I've fixed it, it still sounds great to me. Having all four output transistors and associated emitters on a single die simplified manufacturing and cooling, and they have been pretty reliable. At least they don't blow up any more often than any other output system from the late 70's through the early 80's. Mine blew because a filter cap in the phono amp power supply vented out, and the resultant offset fried the output.
As noted, the only thing really wrong with them is that they are not available from the original manufacturer and replacements aren't as high a quality in many cases. There are new replacements with standard output transistors (in DIY or finished forms) that work very well if you want replaceable components too.
No one uses Darlington pairs any more, they were common to up the output capacity of available transistors back in the day, and with developments in transistors allowing higher voltage and current in a single die, became unnecessary. Not bad, just not needed.
I won't know if it is any good until I get some load resistors and actually do some RMS testing.
