I recently made the same move, from NSP110's to the Alumni's and have them connected to a Yamaha DSP759SE amp.
The Alumni's are obviously a step up, sounding more open, detailed and with much better integration (especially with the sub - which now 'thuds' rather than 'booms' if you know what I mean

). They produce a really good/wide soundstage compared to the NSP's and are far better with music. Surround sounds are better 'placed' (I always found my rear speakers were lacking with the NSP's) and when combined with a much clearer and more capable centre makes for a great/involving movie viewing experience.
I've not read anything negative about the Alumni's apart from the very slight criticism that they aren't the best for music - although I am happy with their stereo playback.
I got mine for £420 inc free next day delivery from here
http://www.petertyson.co.uk/ebuttonz/ebz_product_pages/mordaunt_short_alumni.shtml which to me is a steal for what should be a £600 speaker package. I would suggest you go listen to them before you buy. RS seems to be a popular place for testing them out.
Alternatively, if you are happy with gloss white speakers then there is a great offer for the AE Aego T's 5.1 at £410 delivered - link here
http://www.acoustic-energy.co.uk/E-Shop/index.asp?EnhanceA=PRODUCT DETAILS&RecordID=37