What is the best amp setup for Magnepan 3.6 ?

While not an owner of 3.6's I do own the 1.5's as my 2nd pair of speakers. Presently mated with a Rogue 66 preamp and a Bryston 4B-ST and I think the set-up works great! Given that, my suggestion would be a pair of 7B-ST monoblocks (alternate choice would be a used 14B-ST - 2 7B amps in one chassis and reported better output devices) and a Rogue 99 (tubed preamp - magnum version preferred). You should be able to get both if buying used for your target budget amount. I am sure you will get many other good suggestions which will no doubt offer you much satisfaction but this combo should be auditioned if possible. Bryston is very hard to beat at their pricepoints and have a history of being marrying well with Maggies. Rogue products are a wonderful purchase given their quality/dollar spent ratio.

Used Krell FPB 300 if you really want to see what the Maggies can do. $5k will cover it!
On the other hand you could keep your Rotel and give the rest to the poor. Probably a better choice.

I am using an ARC VT100MkII with my 2.5Rs and I am very satsified. I'd be interested to know if the VT100 had enough juice for the 3.6s. All the Magenplanar speakers benefit from as much power as you can given them. Personally I think the ARC tube amps work well with Magneplanars, but I understand that Bryston is now the amp of choice.