I now own the RE0 and feel they're very good - but not to the point of loving them.
Is there anything with better/same SQ but with a significant increase in isolation? I won't want to spend more than £100 (from the UK) and the choices that I can see with 4.5 Isolation is the ER MC5 (£50), Shure SE215 (£80) and the HiSoundAudio Crystals (£70). At the end of the day I want value so if it's going to be hard to get better value than this then I'll be more than happy to keep the RE0.
Here's a longer post of what I'm looking for: http://www.head-fi.org/t/579877/upgrading-from-the-re0s-for-something-with-more-isolation#post_7886381
All of those easily isolate more than the RE0 but none of them sound as good. unless you are looking for a different sound signature. Listening preferences differ, obviously, but if you're asking my opinion you'll be spending a lot to find sound better than the RE0 with good isolation - think Etymotic HF5 or FIscher Audio SBA-03. Both should be around £100. I'm afraid you won't get away with spending less than that.
The CX300 has more bass, the S2 has cleaner bass but honestly buying the CX300 in 2011 is ridiculous. It's so completely outdated and the prices haven't dropped enough to keep up with the competition. If you just want bass, why not get a MEElec M9? Or, if you want decent overall sound quality, the Dunu Tridents and Soundmagic E10s don't cost a whole lot more.