Among all the other specs, how important is gain in addition to power output at a rated speaker impedance?
Usually, the gain specification of a power amplifier is the voltage gain in dB, which evaluates the amplifier’s amplification capability in terms of the output/input voltage ratio.
For example, the Bryston 14B³ has selectable voltage gain with 23dB (low) and 29dB (high) settings. The preamplifier requires an output voltage of 2.5V (high gain setting) or 5V (low gain setting) to drive the amplifier to its rated power of 600W@8Ω.
In other words, the gain specification of the power amplifier determines the preamplifier output voltage requirements.
I hope I’ve answered your question.