Thanks for the responses. I take it the answer is probably 3 dB, but it might help if I explain the reason for my question. I'm currently building a dedicated listening room that is rectangular, with approximate dimensions of 23' L x 14' W x 8' H. So, a medium sized room of about 2600 cubic feet.
This room is strictly for multichannel music using 4 identical speakers in a layout resembling the quad format of the '70's. 90% of the music channels will be discrete (i.e. 3,4,5 or 5.1 channels) , though I will also listen to some 2 channel music reproduced using Dolby Pro Logic II or DTS Neo 6 to derive 4 channels from 2. There will be no center speaker, but there will be 2 subs implemented with an 80 Hz crossover.
My question about SPL has to do with speaker selection. The speakers I would like the most have a sensitivity rating of 86 dB, but my short list also includes speakers of 88, 90 and 91 dB. Will my preferred 86 dB speakers play loud enough to satisfy me? That's the big question. A couple of days ago I played some music at a volume that I would rarely exceed, while holding in my hand a Rat Shack SPL meter set to C-weighting and fast response. During loud passages, the meter stayed in the range of the mid-80's, with occasional peaks up to 92 dB. I never saw it go higher than that.
For amplification in the new room, I'm planning on a 4-channel ATI Signature class A/B amp rated at 200 wpc into 8 ohms, 300 wpc into 4. My preferred speakers are 8 ohms nominal. The 91 dB speakers are 6 ohms. None of the speakers dip below 3 ohms at any frequency.
So, although 86 dB isn't very efficient, I was hoping that 2 pairs of them would be enough to provide sufficient SPL without audible distortion. I was hoping for more than 3 dB, but maybe that will do it.
If you're wondering, all of the speakers I'm considering are Revels.
Is there a specific reason for using the quad speaker layout? Floyd Toole notes in his latest book starting on page 405 (Sound Reproduction, 3rd Edition) that his research and research by others has shown that the quad speaker configuration is less preferred by listeners.