Hi Mark MG,
I agree - my stuff is in the equivalent of dark storage. BUT that said, if I still see a difference of paper white between stock that is 7 years old and stock that is 1 year old, either it's my imagination (which I'll check with readings), or the surface colour stability may not be all that great - even in dark storage. However, the printed areas still look fine. So ink may cover "many a sin". I think the more important longevity issue for me is what happens to the colours, and less small changes in paper white provided they are even. For B&W photography however it may be a different matter.
The 4900 sensitivity issue - as I've said elsewhere - "it depends" - (a) how bad the problem is, and (b) how much it matters to you, and both factors could be correlated. In my particular case, I'd prefer not to have to bother with nozzle checks and cleaning, but none of it has been fatal, and the quality I get out of this printer is so good that for me the maintenance is not a deal-breaker. This is subjective - will vary from person to person, just speaking for my own experience and priorities. My main focus is the ink on paper - not how the paper feels, so even if these x900s are more sensitive than other models, I wouldn't be scrapping this printer on that account.
I shall check out the Hahn FA Baryta satin - thanks for the suggestion.