I'm in the market for some new monitors with a budget of around £400 (Around $500, because things are expensive over here)
The current contenders are:
KRK Rokit RP5/RP6/RP8
Tannoy Reveal 5As
I'm in a small room, so would it be better to go for smaller speakers + a subwoofer?
Don't forget to put up some acoustic treatment, it's the number one thing to make your monitors sound better, and compared to the price of the monitors it's very cheap, if you can do any DIY at all.
That said I have personally used both of the HS series Yamahas, and they are decent, though a little smeared in the low end and a little strident on the top end. The others I've not used, but I have read good things about the Adams... though if you look around the internets, you can probably find rave reviews and damning criticism of all of them.
Seriously, look into getting a couple of rigid fiberglass bass traps for the room, it will make a world of difference. It would be a much better investment to put up some acoustic treatment first, then to add a subwoofer to your setup.