If you have a RS store near you, certainly worth popping in and see if they have the 220's in that you can demo, along with other speakers within budget. I've also got some 9.1's and it always amazes me when I listen to them, that they cost a mere £100. I've also wondered how the newer Diamond's compare, but it's nigh on impossible finding a comparison of a new speaker against a 12 year old, at least in this case.
The 9.1's were £180 I believe when they came out around 2004, and the 220's are similar price. Although designs move on and improve, given the similar release prices, I would guess the 220 will be similar in performance and might be slightly better. Given the £80 difference, the 9.1's are almost certainly the better buy.
I think for more bass, then you'd probably need a speaker with a larger woofer. Different speakers with the same size woofers can differ in the amount of perceivable bass they put out, as designs will differ. In general though, deeper bass means a larger woofer. Probably around 6.5" for a perceivable difference.
Monitor Audio MR2 are around £140 and have 6.5" woofers. Monitor Audio speakers offer a somewhat different sound signature to Wharfedale though. They tend to have a more prominent treble, deeper bass, but they are more withdrawn in the mid range. Some people prefer this, but some may find them a little cold or clinical sounding. Their discontinued BX2's were very well regarded, and sold for around £240 at one point. Although the BX2 are likely technically better than the 9.1, I have seen people who compared them both when demo'ing, say that they preferred the Wharfedale sound, as it was warmer with better mid range.
Preferences vary from person to person, so that's why it's best to demo if possible when moving from one speaker to another. Buying a completely different brand of speaker without hearing it first, turns out great if the listener prefers the new speaker; but I've seen some people do that, only to end up wanting to return them, because the different sound wasn't to their taste.