I can't see there being any real benefit (if any) in buying a new pair of headphones from what you currently have to justify the spending of £70-100 myself, solely for the low-end.
Save the money which you would use on this slight upgrade, and invest it into your budget so that you have more to use whenever you decide to order the (monitors/interface)
If your finding your current MDR-NC6 which cost £50 to lack the bass for which you feel you need, buying the MDR-7506 that are £60 which are also in the same model range, will most likely be the exact build aside from a minor changes. So there will be no real improvement from what you already have to justify the change.
Again If you really want to change headphones wait till the start of 09' when the sales start, then you should be able to get a good pair which RPP at £150 for your selected budget. Sennheiser are my favourite brand, just make sure that you don't rush into it - as buying a pair now rather than waiting 4-5 weeks will be a bad move in my opinion.