Best improvement for the buck

Consider upgrading your speaker cables. I would advise trying the Alpha Core Goertz MI 3 Divinity Bi-Wire. Goertz has had a known synergy with Magnepan. The Cable Company has a 30 day return policy. A stand alone tube phono preamp like an Allnic H 1200(own one) would bring your viinyl to another level. I do agree room treatments are a must but IMO have your system dialed in then the room treatments should be more effective.

IMO a mediocre horn speaker at $900 new or used would not improve his system, since that is what the OP is after.. A properly set up Magnepan 1.7i rivals box speakers costing ??? you know the rest!