Kevin is exactly correct: there's nothing wrong with NHT speakers, but they are listed as around 86dB or 87dB. The lower the number, the less efficient the speaker. My Paradigm's are around 90dB, but something like the horn speakers used by some folks may have ratings well above 100dB. That's part of why I use a Model 750 (165W per channel) to drive my speakers while Soundhound (for example) uses tube amps with an output of somewhere around 15W -- I forget the exact figure -- to drive his horn speakers.

If you do go add more speakers to achieve a 7.1 system, you would want to have surrounds on the side walls near the listening position (which would most likely be the PSB's you have now) and rear surrounds at the back wall. Many people use the same speakers for surrounds and rear surrounds.

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