Hmm "left field" is not exactly NAP500 or speakers but more like "Ditch the NDS for X or swap the 552 for Y". Neither of which I would suggest just now.
I'd definately get the 300 DRd, or perhaps by a long in the tooth NAP500 (as old as you can find and therefore lowest cost) and then pay to have it recapped and DRed to current spec. Once you move to one of the DR power amps, the next step will present itself obviously.
Is say this from experience that DR does sound very different and offers more of everything but it does absolutely make speakers behave differently. You may find it an improvement in all areas or you may find it (as I did) breaks the spell slightly if your current speakers were carefully matched with your non 300, As soon as you DR the amp, you are effectively using speakers with an amp you never originally matched them to. In my case, a speaker change was required to something more capable of the 250DRs more linear low frequency delivery.
But if "left field" is truly what you're after, fine. Sell the NDS, buy a Revox reel-to-reel and reverse rip everything on your NAS to tape