Hello from the Gold Coast.
I too assume you're talking about AudioConnection. You need to get there and have a listen to them all. Nothing will replace that. But I've got a few thoughts:
* The Purity is amplified so an amplifier will be irrelevant
* The SL3 may need panel replacement. Check the serial number of the panels and then compare to the serial number decoder on this website. That will tell you their age. Now that won't tell you how they've been treated, but if they still sound good and are less than five years old then go for them. If not, be wary.
* Definitely choose the SL3 over the Source and Clarity (if your listening confirms this - it may not). But bear this in mind - the SL3 is a serious high-end model. The others are budget models.
* A NAD amplifier may be inadequate to drive 'stat panels in the long term. Again, listen and confirm this.
Speaking from experience, I tried a Rotel 1090 with my old Aeirus and it was awful! If you listen to the NAD, also try something older/SH from Classé, Krell, Mark Levinson, ARC, et al. IE. Serious stuff. You might get a shock at what a difference it makes!!!!!! It's not about volumes - it's about stability into low-impedance loads.
The preamp is less crucial because it's not affected/connected to the panels. If you like the NAD, go for it.
As an example of outstanding power amp value, have a look on the AudioConnection website at the Classé Model 25 for $4500 or the ARC VS55 for $3000!!
That's not to take away from the NAD - just that 'stat panels are a pretty hard/serious load on an amplifier and something designed to work well with dynamic speakers just won't cut the mustard.
Good luck and enjoy the journey and we might catch up next time I'm in SYD.