I'd have to disagree with PP on this occasion. I've had 11Ls for a long time and have heard them with many different amps. They aren't that different to the 12Ls so here are my thoughts.
Forget Arcam. Its a terrible match. With both an A65+ and an A70 the Quads have sounded dead and vague.
NAD is a better match because the Quads respond well to a high current design, but you can still do better. To me, the C326 and PM6003, whilst both superb, would be more of a sideways step from your existing Rotel which remains a very capable amplifier. In fact, one of the best amps I've had driving the 11Ls was an RA-01, so you may find it hard to improve on it without spending quite a lot more.
My top recommendations if you are determined to change would be a used Pioneer A400, which works brilliantly with the Quads, or (if you can stretch) a new Yamaha A-S700. Both will effortlessly deliver the power the Quads need without any signs of strain, congestion or woolliness.
Don't rush into it though. Your Rotel is a superb piece of kit and if you are enjoying the sound.....