You wouldn't lose any midrange either way. In fact with either of the Studio series the midrange will be much improved.

You will lose some low frequencies regardless, although with the 40's it would be negligible. Paradigm's website is not right I don't think, they list the Studio 20's as rolling off around 52 hz which is fine, but they have the 40's rolling off at 64 hz. That can't be right. The 40's have double the drivers and double the enclosure.

You have to keep in mind that the Monitor 9's use larger drivers and a much larger enclosure, and you can't beat physics. They are going to have more low end. But if you have the 40's mated to a good sub then it won't matter.

Personally, if you're looking for sound quality I think the 40's on proper 24" stands are hard to beat. The difference in sound quality alone would be worth the upgrade.