>>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.
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.