Speaker Effeciency and Bass Production


How much speaker efficiency is gained if the bottom two octaves or so don't need to be produced? I'm considering trying an SET or OTL amp but am wondering about the ability of these amp types to drive my speakers (Opera Quinta; 89db efficiency and 6 ohm nominal impedance; 6-8 ohms from 100Hz up). I am using a pair of corner placed subwoofers which can easily produce the lower 2-3 octaves. So the question is if the amps driving the main speakers don't have to produce anything below 100Hz or so, then is the ability to use them on my main speakers more doable? The room is fairly large, but I sit only around 10 feet from them.
smeyers
Much will depend on the volume level you desire. For instance, the setup Myrtle described will only produce a max 93 dB at the listening position; Bob R.'s setup can do 94 dB. Use this SPL Chart to determine what would be an appropriate level for you. As a minimum you need a +10dB capacity above average levels for dynamic peaks.
Thanks much for the responses! It is interesting to note that so far I have used 3 different sets of tube amps so far on this setup with no as yet noticeable problems (although I've yet to push it VERY hard):

AES Sixpac: 50W push-pull triode
ASL Wave 20: 20W push-pull
Wright Sound: 10W push-pull triode

I do wonder if there is a formula than can calculate increase in efficiency based on low frequency cutoff. In other words if an amp can drive speakers full range to 94db in a room just before a certain clipping distortion, then if the amps are high-pass fed the same signal at a certain frequency (lets say 80Hz), then what SPL can the same speakers in the same room be driven to the same clipping distortion?
Onhwy, thanks for the link to the chart. According to that chart, I would need approximately 30W or so at peak levels; although isn't calculating efficiency a little more complicated as Bob_reynolds suggests above?
The answer to your original question is NO. Adding a subwoofer won't change the speakers sensitivity. Removing low bass frequencies will lessen amplifier clipping (on most types of music), but it won't effect the maximum volume level attainable by the amp/speaker combo. So the system could sound cleaner, less distortion, but it won't go any louder.

Calculating max SPLs is fairly simple. Matching a speaker's impedance to an appropriate amp is more complicated. You need to be more specific about the amps you're considering to get more detailed and better replies.
Usually when a subwoofer is integrated into a system, the lower octaves are not going to the speakers anyway. In my setup, both my pre amp and power amp are connected to a powered subwoofer and the speakers are connected to the power amp, not the subwoofer. In a sense I am biamping the subwoofer, but the lower frequencies are not produced by the speakers. My speakers are very efficient 92db and are rated to about 35-40hz in the lower region, but the lower frequencies are produced by the subwoofer. The speakers are producing anything above 100hz.