Speaker Efficiency Question?

Assuming that your tube amp has output taps for 4 ohm speakers, it will deliver the same power(watts)into 4 ohms as in to 8 ohms. But that is not really good, because the typical ss amp would deliver twice as many watts into 4 ohms. Loudspeaker SPL ratings these days are usually given for a certain voltage applied to the speaker. The voltage used is 2.83 volts and this corresponds to 1 watt for a 8 ohm speaker and 2 watts for a 4 ohm speaker.

So, to answer your last question...4 and 8 ohm speakers with the same SPL spec will play equally as loudly if driven by a ss amp, because the ss amp will deliver the extra current required for the 4 ohm speaker. If a tube amp is used, the extra current must be provided by using a lower tap on the output transformer, which means that output voltage swing has been traded off for current. Therefore the output voltage capability, which defines how loud the speaker plays, is reduced. So, in your case, you will see lower SPL with a 4 ohm speaker. But it might be ok anyway.

I think the correct figure is about 3dB. You will get about 90dB/w/m with the Tyler Acoustic speakers. Subtract about 2-3dB per meter after that. At 6'-8', you will get approx. 88dB/w/m. I wouldn't think "effiiciency" is too much of a consideration unless you like to listen really loud.

However, don't forget unless speakers under consideration have a relatively flat impedance plot, your Thor amp (which is tubed) may not be able to drive well at less than 4 ohms.

See http://www.welbornelabs.com/recomendspeaks.htm & scroll down about 2/3 for a good article on proper speaker selection with a tube amp.

see Rabelais' responses

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1108496641&read&3&4&

>>I was told that if a speaker is of the 4 ohm variety that you have to reduce the efficiency rating by 5 dbs.
Well, it depends........
On what?? It depends on how the manuf. spec'd the efficiency. Did he say 93dB/1W or did he say 93dB/2.83V?? If he said, 93dB/1W then it doesn't matter whether the speaker is 8Ohms or 4Ohms 'cuz 1W is 1W & in an 8Ohms speaker it'll create 2.83Vrms (as Eldartford wrote) & in a 4Ohm speaker it'll create 2Vrms. In this case, take the efficiency value as correct.
If he wrote 93dB/2.83V then you'll have to reduce the efficiency by 3dB ('cuz the speaker manuf. "cheated" by inputting more power - 2W as Eldartford wrote - into the speaker. Naturally, if he doubled the power, he's going to get a better efficiency!)

If the efficiency is 93dB/1W then you might be compromised by your Thor amp when running it off the 4 Ohms tap (if it has one) since the voltage swing is reduced => music amplitude into the speaker is less.
If the speaker efficiency is 93dB/2.83V then you'll have to subtract 3dB & you'll about 2dB better than your Quads. 2dB is a reasonable (58%) improvement in that you need this much less power from the Thor. Might just do the trick for you depending upon what you listen to & how loud.
FWIW. Hope that this helps.