Hi Bev.
Geoff's advice in post 79 is spot-on. That's where I am with my main system, and it seems like you're almost there, too
IME, Naim have really nailed their amp 'ladder' at the moment. I've heard most of the current range, and each is really very good at sounding 'just right' in isolation / at each step ... which, as you've discovered, makes auditioning rather challenging, because they all sound good when appropriately matched
The Nait 5si I had sounded very engaging and balanced, when fed with a CD5si and driving a pair of Tannoy Revolution XT6 stand-mounters. It wasn't until I got the CDX2 and a pair of KEF Reference 104/2 speakers that it started to struggle a bit, and its limitations became more obvious. I went on to a Supernait 2, but your speakers are easy to drive, so I'm not surprised that each amp sounds fine in its own right.
From my experience the SN2 will probably drive most speakers properly, so maybe if you think that you might ever change speakers in the future, it might be a safer long-term bet? It also has enough resolution to clearly show any source upgrades, too ... the differences between the 3 cd players I have atm were more easily heard with the SN2 than the Nait 5si. If all else fails, just disengage your analytical brain and follow your heart