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.
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.