Hi Musicnoise,
have you decided on a budget? I have read many of the opinions above and of course they swing many ways. From a Mackie PA to a quad electrostat...!
(by the way the mackie is very good as a PA. Their little studio active monitors are sweet too)
Box speakers are always going to give you problems though, and IMHO a bad place to start if listening to classical music only.
Many people have mentioned planars like Magnepan, Quad, Apogee etc including myself. All great, but Quads will give you problems with light bass and flat dynamics if listening to large scale works. Magnepan and Apogee will need a lot of power, but will give you beautiful bass, detail beyond your dreams and excellent soundstaging. In fact problably the closest to being in a concert hall experience for the money and maybe any money.
I also mentioned the Avantgarde Trio with bass horns. Very expensive but if you have the room and the cash, would probably end your search as they have an intimacy and dynamic contrast few can match and certainly would kill every speaker I know in palpability due to their high sensitivity. The cheaper models from Avantgarde are good too (Uno/Duo etc). Avantgarde are the kings of dynamics. They are utterly effortless and are almost as transparent as the planars mentioned above.
Many classical recordings are made using B&W 801d or 802d as monitors. A very good speaker but again needs lots of power to come to life and lots of money to buy. I have used them tri amped with 1,200watts/channel.
To help you we really need to know the size of room and your budget otherwise you will be getting all sorts of advice that is useless.
If you have very little budget buy an old Magnepan. They wont let you down (Not too old though!).
Good luck