Budget amp that will give Magnepan 3.6Rs better depth of soundstage

I would try hanging  a curtain  behind the speakers about three inches away from the wall. You want sheers or better yet a sheers with small holes in them. That will eliminate  your muddy  part likely. Very cheap to do it helps control  the back wave ant the top end makes it much clear. 

I ran Maggie 3.3s actively  bl amped with s pair of moon w3 amps. Lots of power grip speed. Huge sound stage. 

I have a bit of good news for you. If you can be patient, and stretch the budget a bit, find a Music Reference RM-10… 35 watt per channel tube amplifier. They are usually around $1000-$1200 used, so definitely north of your budget, but they sound absolutely incredible with Magnepans…. As long as you’re not trying to play music at 100 dBs. Your 14x14 room is helping you. A number of people will likely chime in and tell you it won’t work, but it does work. I’ve owned six different models of Magnepan speakers over the past 37 years, and I’ve tried dozens of amplifiers to drive them, and I can tell you with absolute certainty that the RM-10 I’m listening to right now is the best sound I’ve heard in my room. My room is 14x22, and I listen mostly to rock, blues, and some jazz, at an average of 80-85 dBs, measured with a sound meter, at ten feet back from the plane of the speakers (they’re about 52” out from the front wall). My speakers are MG-3.7s. I’ve been experimenting with lower powered, high-quality amplifiers with them over the last couple of years. Those who insist Maggies “need” hundreds of watts of power are either using the wrong amps, or have completely different priorities than I. By the way, I’ve used big, heavy, monstrous amps capable of outputting several hundred watts per channel with these speakers. Some have sounded very good, some not so good, but sound quality, for me, has never followed a “watts per channel” model. *Caveat: I recently purchased a pair of zero autoformers from Speltz audio to increase the 4 ohm impedance of the speakers. Roger Modjeski, designer of the RM-10, assured me, before he passed, that his amp would be up to the challenge of driving Magnepans to the level I listen, with the music I listen to, in the size room I’m in. He was correct. 

A used Acurus A 250 is within your price range and will give you at least 250wpc into 8 ohms. But will it be of sufficient quality to get the best out of your Maggies is a question I can't answer.

I agree with misstl, Maggie 3.6s are a lot of speaker for your room. So, making the room ’seem’ bigger acoustically would be the logical first step. That means acoustical panels. They will reduce the level of reflected vs. direct energy and in doing so increase the Critical Distance, that point at which direct and reverberant field levels are the same. My personal preference is Acoustimac for construction quality, fair pricing, and a wide selection of fabric options. Their Home Theater Package 1 would be a great place to start in your room.

Don’t believe that spending more on tweakier panels is money well spent. Inside all of them is either Corning Fiberglas 703/704 or Rock Wool or a paper based material called Eco Core. Rock Wool is the standard works just fine. Much as I believe ceiling panels area Very Good Thing, because Maggies are dipole radiators, I would consider trying 2 of the 6 panels or mounting the bass traps behind the speakers to help control reflections from the back wave.