Boy, here's the old tube watt versus solid state watt question.
Tubes clip softer and distort in less objectionable ways, so they can effectively be driven harder than solid state watt for watt. Rules of thumb put this as high as double or triple as much.
And a chance to expound on how a watt translates into dB.
Recall that one dB is the smallest incremental increase in sound pressure level (spl) that the average person can detect. Power (wattage) relates to spl's logarithmically, so to double the dBs (to sound twice as loud) you must increase power by a factor of ten. So in the example above 2 or 3 times the power is only about 4 dB louder (a bit more than half again as loud). Watts can be converted into dBs of gain then added to rated speaker efficiencies to calculate how loud any given amp/speaker combination can theoritically get. Speaker efficiencies are stated in terms of xx dB/w/m where "w" is watts, more specifically one watt input.
Here's a table of watts to dB of gain:
1 watt = 0 dB 10 watts = 10 dB 100 watts = 20 dB
2 watts = 3 dB 20 watts = 13 dB 200 watts = 23 dB
4 watts = 6 dB 40 watts = 16 dB 400 watts = 26 dB
5 watts = 7 dB 50 watts = 17 dB 500 watts = 27 dB
8 watts = 9 dB 80 watts = 19 dB 800 watts = 29 dB
Example 1: 85 dB/w/m (low efficiency) speaker and a 100 watt amplifier = 85 + 20 = 105 dB.
Example 2: 90 dB/w/m (medium efficiency) speaker and a 40 watt amplifier = 90 + 16 = 106 dB.
Example 3: 95 dB/w/m (high efficiency) speaker and a 10 watt amplifier = 95 + 10 = 105 dB.
105 dB peak spl is a fairly good target to shoot for in home use as 25 dB is a ballpark amount of headroom to count on for dynamics in most music and 80 dB is quite loud for average playback levels.
So using the example at the beginning of this thread: 105 - 13 = 92 dB/w/m efficiency speakers.
BTW I own the first pair of the FTA-2000 speakers mentioned above that are 90 dB/w/m efficient, so I'm looking for 40 watts per channel (two channels add 3 dB, but losses in distance, plus gains from reflection in a 2000 cu. ft. room that I'm using roughly balance all this out). The FTA-2000 cover 99% of all music, provide a direct amp to driver connection, offer the best possible imaging source, eliminates phasing issues, crossover distortions/losses, transmission line benign load to the amp with clear midrange, and does it without a whizzer cone. It's sound is detailed, but more so it is musical so that well recorded material is rewarded while poor quality material is still enjoyable as they always put the music first.
