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

Give nice Class D a try? You can get a Hypex NC502MP based amp new for that much. It's 500 watts which your Maggies will enjoy, especially for transient peaks. Suggested brands would be VTV and Audiophonics. I use a March P502 with my MMGs, and they've never been driven so well before for me. Incredible clarity, detail and drive. With Magna Risers, my MMGs have supplanted my Vandersteen 2ce Sigs as my faves. 

Mine are actively biamped with a Marchand crossover and a Classe CA-200 (treble, mid) and a Nakamichi PA7.  Active biamping is really the way to use budget amps; Maggies are very easy because of the external crossovers.

Look for an amp that doubles watts from 8 to 4 ohms.  That means good high current.  Don’t listen to anyone who suggests bridging an amp into your Maggies.  Not a great idea with most amps to bridge into 4 ohms.

budget amps used with decent success on my Maggies:

Krell KAV 250a - doubles into 4 ohms and stays cool

Parasound A21 - also muscular 

Levinson 23

i also love the Proceed HPA2 as suggested above.  

Maggies are incredibly sensitive to placement and toe in angle.

I've experimented with several sub $1k power amps with my one-year-old Maggie 3.7i pair, which should have similar power characteristics to your 3.6R speakers.

Here are some of my observations which of course are entirely subjective-

An older Conrad Johnson Sonograph SA-250 amp did a very nice job of bringing out the big, beautiful Maggie sound.  It is not as detailed as my newer, much more expensive Bryston 3B3, but well worth a try for about $650. I would say it has about 80% of the characteristics of the newer Bryston, with to my ears, no significant problems with the Maggies.  If I hadn't bought the Bryston, I would keep the Sonographe.  I will be selling it soon. The is also a SA-400 model out there occasionally for sale.

I also tried a $550 B&K EX-442 which has higher rated output to the SA-250 above, but to my ears did not control the Maggie bottom end well.   Sort of muddy and indistinct sound with the big panels

Still one of my favorite amps with the 3.7i pair is the vintage Counterpoint tube hybrid SA-100.  It has a distinctive transparent and live sound, maybe a little shy on heavy bass, but overall, extremely natural sounding with the Magnepans.  They can be found between 500 to 1000 dollars but be careful that it is working as they are difficult to fix.  I have one and don't intend to sell. This amp brings out natural instrumentation the best, like unplugged piano, guitar, orchestral music and voice.  The piano sounds like the piano is in the room. The instruments and voice sort of jump off the panel.  I would use this amp only but my newer Bryston 3B3 is more well-rounded in other respects.

Although they are more expensive than your price range, I've tried both the Rogue Pharaoh integrated and the Rogue Stereo 100 tube amps.  I didn't like the characteristics of either of these units with my 3.7i set.  The Stereo 100 tube amp was not necessarily underpowered but created a kind of filmy overlay in the base/mid base that to me was objectionable.   Maybe this was some tube that creates the "tube" sound, but to me it was a problem with the Maggies. The Maggies need an amplifier that can grab the panels and make them sing with control, not add coloration. Maybe it is not as noticeable with traditional cone speakers.  

Again, this is all subjective but maybe helpful to you as a fellow Magnepan owner. 😁 

Three things going on.

Room is too small for panel speakers.

You’ll likely not find an amp that has high current AND great sonics in your price range.

Magnepans are never going to have a deep soundstage. It’s the nature of the beast. They do what they do well, but aren’t all things to all men.

I say this as a former Maggie owner.

I would look on Audiogon or another site for any vintage Classe Amp with 200 wattsper channel. CA200, CLASSE 15. something like that. You might get lucky. 

They are good power hungry panels, but they do reward you when you feed them well.