I built a SSE some years ago and am still using it in a highly modified form, but it's about to be replaced by p-t-p wiring and KT150 power tubes...
In any case, the pcb is a little on the thin side for r&r'ing tubes but they don't tend to break and in both the SSE and TSE there are layout issues as far as hot parts being placed too near to caps. Other than that great amps and lots of support with an entire tubelab forum over on diyaudio.
Also, if you build a KT88 SET buy 2 sets of cathode resistors and some EL34s too, I like the EL34 better... more natural and less hi-fi sounding in my system.