Sounds worse than a 310???
Yep, much worse. I know, been there, heard them all.
If you go for the RCD/RNS510 you'll need a sub-woofer and a seperate amp. So if you go this route of RCD/RNS510 go with the Dynaudio as it's good value. But you'll still need a sub-woofer.
Did not feel the need for a sub-woofer or amplifier with the RCD310. And just one more thing, the RNS510 sounds very slightly worse than the RCD510.
EDIT: The RCD310 is not very pretty and I'm glad I got rid off the eyesore.