Bookshelves

Make sure that they don't have a rear port.
You could even go for something little in a sealed enclosure like gallo's with a sub on the floor somewhere?

LS3/5A's embody the soul of your request. They don't get better than this, that is why they are still popular 30 years latter! I've been responsible for a lot of friends buying them over the years, and most still have them, and just love them. BTW, I think the Rogers and Spendors were best at what you are looking for. Some of the latter ones started to 'enhance' the FR and some actually use different drivers, they are really nothing more than LS3/5a look alikes, not sound alikes.

Of course you must use front ported speakers. I agree LS3 is the way to go.

I'm a bit confused by the LS3/5a suggestion. While they are small and would fit on a bookshelf, the OP asked for a "thin" sounding speaker. I'd consider the LS3/5a to have anything but a "thin" sound. I've always found them to have a rather rich, textured and articulate sound, though absent deep bass due to their size.

That said, I do love the speakers (I have two pair of other model Spendors plus LS3/5a clones) and do consider them extremely musical, especially if one is fond of music primarily played on acoustic instruments and with unprocessed vocals.

Perhaps the OP could elucidate a bit more about the type of music he prefers and his volume requirements. In re-reading his original post, I'm not quite sure what I'd suggest at this point.

It seems like you are asking for a neutral speaker more than anything else, with perhaps more emphasis on detail than bass or midrange warmth, and not being annoying in the treble? I'd look at the Polk monitor 40's, which are very dry but enjoyable, and retail for about 2-300. If you want to spend more money as you suggest, the Aerial model 5's may be a good choice.