I recently auditioned all three of these speakers (and ended up choosing the Monitor 11's), but I liked the Studio 20's quite a bit. They put out a whole lot of really good quality sound. I thought that the Monitor 7's were very good, just a little weak, though they would certainly benefit from a good sub. The Monitor 9's have two 8 inch drivers (compared to the two 6 & 1/2 inch drivers in the Monitor 7's & the 7 inch driver in the 20's) that I thought gave them a flabby sound, and I wouldn't recommend these at all. For the money, I would go with the Studio 20's hands down.
The salesperson switched to the Studio 20's when I was doing side by side tests of the speakers, and it took me a moment to correctly identify the source of the sound. Compared to the size of the speaker, the 20's sound huge. If you listen to the 20's and still think that they're missing that bass driver, the 40's offer it for just a couple hundred more bucks. However, I think that you would be the most satisfied with the Studio 20's (but, of course what matters most is what YOU think about them, so I would just give them a listen and decide for yourself)!
-Stephen
