Even if you get a new receiver, I don't know that my recommendation would change.
If you can, find a dealer who sells the NAD AND Arcam and ask to audition the speakers through each respective receiver.
I haven't heard either one of the receivers that you list, but I have owned integrated amps from both NAD and Arcam and I was more impressed by the musicality of the Arcam. The Arcam with a pair of Quads might be pretty nice. You've gotta' listen and find out.
Finally, regarding the Quad center channel. I wouldn't worry about it. It's guaranteed to use drivers matched to the 12Ls. If you like the sound of the 12Ls, then the matched center should be equally good.
Keep us "posted".
If you can, find a dealer who sells the NAD AND Arcam and ask to audition the speakers through each respective receiver.
I haven't heard either one of the receivers that you list, but I have owned integrated amps from both NAD and Arcam and I was more impressed by the musicality of the Arcam. The Arcam with a pair of Quads might be pretty nice. You've gotta' listen and find out.
Finally, regarding the Quad center channel. I wouldn't worry about it. It's guaranteed to use drivers matched to the 12Ls. If you like the sound of the 12Ls, then the matched center should be equally good.
Keep us "posted".