As you can see, we manufacture a very comprehensive range of vacuum tube power amplifiers encompassing models from 12 to 500 watts. Why this large choice of power capability, you may well ask...? Three technical (and one emotional) parameters apply here: Firstly, the sensitivity (sometimes wrongly termed 'efficiency') of the proposed loudspeaker is crucially important. Secondly, the size or volume of the room plus its inherent sound absorbent materials (curtains, drapes. carpets, soft furnishings) comes into play. Thirdly, the SPL (Sound Pressure Level) averagely required by the listener. Those are the major scientific aspects. The 'emotional' one really gets down in human terms to: Does one want to play driving rock and roll and feel that kick drum pounding or attempt re-creation of a full symphony orchestra in one's home environment? (Landlords and neighbors have a lot to do with that answer!)
Using an average room size (in USA terms: 20' X 15' X 9') one can use some "rules of thumb" generalizations: Loudspeakers of 96 to 100+ dB (@1W/1M) of sensitivity can provide sufficient or averagely acceptable listening-level (SPL therefore) with amplification-power from 15 to 50 watts. Speakers of 92 to 95 dB sensitivity will require 75 to 150 watts or more while loudspeakers under 91 dB will need 200 watts and upwards.... If the amplifiers are to run with sufficient headroom before power clip or major distortion comes to bear.
The "Golden Rule" (if there is such a thing!) is that more power ALWAYS beats less power. The best sounding system will always have an excess of power (or very high sensitivity loudspeakers or both) so that even heavy transients are comfortably accommodated without the amplifier even nearly reaching its 'overload' or 'clip' maxima. PLEASE TRY ALWAYS TO CHOOSE THE RIGHT LOUDSPEAKER / AMPLIFIER COMBINATION AND ERR ON THE CONSERVATIVE SIDE (EXCESS POWER) WHERE POSSIBLE.