The only speakers in that batch I've heard is the KRK 5's. I thought they sounded surprisingly good for the price, but I didn't have anything to compare them to, and wasn't mixing on them, which is all that really matters. My guess is that in that price range there's not going to be a lot of difference, and that includes your M1s.
What are you not getting from the M1's? The only role of a recording monitor really is to let you create a mix that sounds good on other people's speakers. Even if they sound bad themselves, as long as you can get the results, that's ok, oddly enough. Yahama's NS10s are widely used in studios, and are basically famous for being so bad that if you can make your mixes sound good on them, they're sure to sound great everywhere else :-)
In that price range, I think the idea of headphones or in-ear phones is not a bad idea, if you're looking for flat. Rather than new monitors, you might consider hanging on to the M1s and just getting a good set of phones.
Here's an interesting article Fran Guidry pointed me to some time back. Read all the machinations this guy's going thru to get a flat pair of reference ear buds, then scroll to the end to see that he's happy with just a pair of Shure EC2s, under $100.
http://www.linkwitzlab.com/reference_earphones.htm