I've spent a couple of hours with MR5 and RP5G2 side by side in an wide open area with carpeting. RP5G2 sounded very good, treble lacked a bit of presence and extension for my tastes and mids/bass were rounded and warm, a pleasant reddish-orange color.
MR5 was slightly clearer with better soundstage and very flat with much better treble and slightly more detailed all throughout, and very colorless, too - it showed me the color of my portable rig only and none of its own mixed in. It was not the first time I heard them, but they definitely sounded like they were a lot more broken-in than the first couple of times in other stores. Both had clear bass with good response. I think the KRKs went a bit deeper down, and they are bassier than the Mackies.
I heard HS50 and BX5a in a Guitar Center, but the area was more closed in and everything had funky balances, even MR5's and RP5G2's that were there. HS50 had an impressive sound. I didn't like HS50's coconut milky-white tone, and the young guy in charge of the section recommended them highly. He said they have the best tone for monitoring. Yuck. HS80 was even more impressive, but had that same tone yet smoother. BX5a had real nice clarity and very good soundstage and imaging ability as well. There was RP6 and MR8, but the MR8's weren't working and the RP6 didn't seem like it was fully broken-in, unless it sucks that bad compared to HS80 and even RP5G2.
I like BX5a and MR5 the most of the bunch, and I enjoyed RP5G2's more musical and tuneful sound, too. They certainly didn't sound bad to me.
Would've been cool if I could have borrowed them to hear them out of my desktop rig. =P
I'd recommend you go with whichever you like best of the monitors you mentioned.
Someone else will have to chime in on anything that beats these for the less money.
People have posted impressions on the MR5's, BX5a's, KRK's, and even the AV40's. Do a search.
Edit: I should mention why I was happy with all of them, lol. I heard some random Creative, Logitech, and Altec Lansing computer speakers with subwoofers and multi-channel and all, very close to the the same prices, and they sounded like **** compared to the monitors. I forgot what models the computer speakers were. (Probably not a fair comparison in the first place, lol.)
Edited by Mad Max - 8/16/10 at 2:36am