I believe that the reason most bookshelf speakers are 5.25-6.5" is to keep the physical size down and because they are often paired with a sub. I believe that Jack (the OP), since he wants a bookshelf that will go down to around 45hz, would be best served with larger enclosure and a larger driver. 8" is pretty rare, but 6.5 isn't.
Jack, if you're interested in checking out Polk bookshelf's, the RTi A3 ($400/pair) are pretty good. Plus they are well within your price range. If your receiver can handle 4ohms, there is also the Polk LSi9 ($1000/pair), which has dual 5.25 drivers and is noticeably better (clearer, richer mids, more punchy, tighter bass, more natural highs) than the RTi series.
There is also the PSB Imagine B ($1000/pair). It doesn't dig as deep as some of the models discussed earlier, but is a really nice sounding speaker. If you want to save money but want a PSB, there is the Image B6 ($500).
There is also the NHT Classic Two and Three ($800 and $600/pair, respectively). They sound fantastic for the money, but are pretty difficult to drive.
Quote:
Originally Posted by
mibutenma 
I really don't know why the speaker has to have 8 inch drivers most really good bookself are 5 and half or 6 and a half.