Hi!
So here I go. I´ll write like a report to be easier for me to write.
Before buying my system I tried at home MA GX300 and Magnepan 1.7 in real time comparison.
Room positioning:
I´ve a dedicated listening room, so I could choose to position the 1.7 wherever I felt it sounded better; in this case against the short wall. If I was limited to the long wall, I wouldn´t have bought them for any price, but I´d stick with the GX300 with no doubt. This description to point the importance of the placement in a room and that you shouldn´t buy a pair without a audiotest "not at you home" but "at you audio room".
Subwoofer (sub-bass system):
Gx300 are much easier to set in place than 1.7 and are fantastic speakers. But, in my room, 1.7 plus REL T7 sounded a bit better for the same price.
I can´t think of 1.7 without sub and not only for the bass.The REL sub improves bass, and is determinant to the overall sound of the 1.7, removing some constraint, but expendable with GX300 as you may well know.
One more time, positioning a sub isn´t always easy.
Amp:
Your Primare amp is good for GX300 and can make 1.7 sing well, but it´ll be on their limit. I´ve a Bryston 4bsst2 that suits them much better.
However you´re thinking of Magnepan 3.7i...
I compared the 1.7 with 3.7 just for fun as the last are much more expensive. First thing that stands out: the 3.7 are even more dinamic than 1.7, but even much more power hungry. Neither your Primare nor my Bryston could drive them with "enough breath".
In conclusion, In my opinion you have a wonderful system and if you have the itch for maggies, try them at home. If you go for the 3.7i, you can use your amp for sometime and, with time, choose something you like similar to Bryston 7b3.
If I was buying a system now for my audio room, I´d buy again Magnepan+Rel, but if I was buying for any other room at home, I´d probably choose something like MA gold300 or Proac of simillar range.
Hope you find my experience useful.
Please ask, if you think I can help.