I'd forgotten about the Momentum. They do look very good, if a little pricey; but then Sennheiser has upped the price of their higher end products. The HD650 was about £180 six years ago, now they are about £280. £335 on the jungle site! My word. Now that is a monster price hike. As good as they are, I've have seen comments from some who own them, think they are not worth £280, let alone £330+. Those that paid £180 a few years ago, must have smiles on their faces. 
Not looked into the MDR-1R at all. £160 on the jungle site looks interesting indeed. Trusted Reviews, had them priced at £300 and they are £300 on Sony's own site.
Now you say you don't need much bass, but if that were the case, then the K550 would have appealed more than they did. I assuming you got a good seal though, when I say that, as that is paramount. No seal with the K550 will result in poorer SQ.

Not looked into the MDR-1R at all. £160 on the jungle site looks interesting indeed. Trusted Reviews, had them priced at £300 and they are £300 on Sony's own site.
Now you say you don't need much bass, but if that were the case, then the K550 would have appealed more than they did. I assuming you got a good seal though, when I say that, as that is paramount. No seal with the K550 will result in poorer SQ.
That chaps review on Head-Fi makes it sound as if they're pretty poor, but he seems to be alone with his sentiments, which makes me wonder if he perhaps had a dodgy pair.
They are very bassy though, more so then the D2000 it would seem.
Best to try what you can first. Ideally it would be fantastic if one could walk into an audio shop and try any headphone. Sadly it isn't as easy that.
They are a fairly reasonable sounding £650 though.