The speaker manufacturer decides this. But Gordon's helpful response to this query in post #107 explains why a 25W tube amp might work as well as a 100W SS amp for any particular speaker. His points are that tube amps can run closer to their max output before clipping, and a 3dB volume difference does not sound much louder. 'So, if you have a 25 watt amp which you can "use" to 24 watts, vs. a 100w amp that you can only "use" up to 50- that's only 3dB difference. Not that much perceived volume difference at all...' .