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.
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Impossible to answer that one definitively but if it helps, I use a 10w amp to drive 90dB speakers in a large room, sitting 10 feet away and I have never heard my amps clip. I don't listen at ear splitting levels but cranking it up my ears run out of head room before the amps do.

No doubt that removing bass below 100Hz will help your amps, but a 5 to 8W amp may surprise you with those speakers, even without the subs.
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.
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.
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.
Onhwy, I don't know specifically yet which amps I'm considering, but thinking about some type of SET or OTL amp. Both can be attained with fairly high amounts of power, but I don't want the complexity or power consumption of anything above say 50 watts or so. A couple of amps I noticed were ASL Explorer 805 and Atma-sphere S-30 (I am concerned about the Atma-sphere's ability to drive my speakers due to impedance).

"So the system could sound cleaner, less distortion, but it won't go any louder."

Yes, the question is how much more *clean* non clipping volume can be had by not pushing the main power amps to provide low frequencies?

Cyclonicman, what are the amps that are powering the main speakers, and is it enough power?
Good point!! It is a Vintage Conrad Johnson Premier 1, about 20 years old, but very good sound and power, 200 Watts per channel, which you aren't going to get with the SET/OTL. However, I was also using the same setup with a Rogue stereo 90 in Triode mode, which was about 40 watts per channel. The powered subwoofwer has a 175 Watt continuous amplifier and 475 Watts dynamic range.
You will probably need about 20 watts with those 89db speakers. The rule of thumb is that your speakers should be able to produce 101db at 1meter with the amp that you will use, and 89db speakers will require 16 watts to do that. But it is only a rule of thumb and other factors can come into play such as- size of istening room, and how complex the crossovers are in your speakers (complex crossovers can eat up power) and what type of music you listen to (orchastra music will require more power than simple music with few instruments playing) and lastly, how loud you like your music to be. For single ended try a 845 output tube (Antique Sound Labs makes a descent one) if your thinking about OTL call Ralph at Atmosphere and ask him what he thinks and don't forget Transedent Sound OTL amps.
Cyclonicman, there are OTLs that can make 500 watts, FWIW.
Really, probably pretty expensive?
Atmasphere, I was curious and so I just viewed the MA-3 monoblocks which as you said can deliver 500 Watts. An amazing piece of hardware, it looks bigger than my furnace.
Thanks Racamuti. I believe the crossovers in the main speakers (Opera Quinta) are relatively simple, and I am doing the crossover between the subs and main speakers in the digital domain (using a Tact 2.2XP). I listen to a range of music, but singer-songwriter music is what I listen to most. I do also enjoy the occasional 100db+ blast of a good rock or classical tune, but it is more the exception. The room is fairly large, but I sit only about 10 feet from the main speakers. I had been considering the ASL amp (as mentioned in an earlier post). The little ASL Wave 20's have been the most reliable tube amps that I have had, and have not even needed a tube change in over two years of 10 hours per day use in an office system.