I was warned off CX300's as i was looking for budget entry level phones with a punchy bass and i didn't want that horrible flat/muddy bass you get with a lot of the cheap phones.
Mark2410 suggested the Radiopaq Jazz to me (£19.99 direct) and they just arrived today, only 2 hours into the burn-in but they have a nice tight/punchy bass and a good overall sound quality. (i listen to Metal, Rock, Country, 80s synth, classical, all sorts of stuff really) So far for the tracks i've tried they are doing a grand job.
I'm sure for the extra ££'s i'll notice their flaws compared to pricier phones but for this price point i have to say i'm quite impressed.
So my personal opinion, if your in the UK get some radiopaq jazz on your shopping list