A typical gain of a power amp may well be between 20 to 30dB, and if its I/P sensitivity is about 0.5V which is typical, that would produce an O/P voltage of approximately 5 to 15 V, 10V across 8 ohms giving an O/P power of about 12W, and 15V about 28W.

That all seems reasonable to me.

I've been pondering the possibility of a feedback loop which had become open cct, and has resulted in a jump in gain.