Has the sound actually deteriorated, or are you just outgrowing the 2.5s? My guess is they sound real similar to my 3.3s, maybe even better considering the smaller room I am using them in.
I have owned my Maggie MG3a's for 26 years now. I used to have a GFA 555 amp, which had just enough muscle to drive them. In the early 90s I switched to a smaller tube amp and got the NHTs. It was just enough to drive the NHTs in a smaller room, and the Maggie's started sounding real dead. I thought they just got old.
Then a few months ago I bought a pair of monoblock amps from Emotiva. They put out 400 watts into 4 ohms. I hooked them to the Magnepans just to see if they sounded decent and was floored. Once I got the Maggie's out into a large room with lots of space they simply blew me away. I listened to music for a couple of weeks with a smile on my face. Like some kind of audio idiot.
With an upstairs and downstairs system, I can take the music back and forth between systems and listen to how different they sound. The Magnepans do some kind of magic of throwing the sound of a real instrument or person into space. It isn't just an imaging thing though. Lots of speakers image well. I think it is the dipole nature of the sound just seeming more realistic and solid to me. The best way to describe it is that a good box speaker makes it sound like the sound comes from there, and Maggie's make it sound like the instrument is there. Bass is also extremely realistic, definitely quality over quantity though. Mine are flat to 30hz if not driven too loud (95db or less?)
I do think I am just a sucker for dipoles. At the recent Chicago audio show, I preferred the rooms with electrostatic and various panel speakers over most of the boxes. Regardless of cost.
Don't even consider Magnepans though without gobs of power and lots of space. The bass and soundstage came alive for me after I get them about 4 feet away from the rear wall. After my 26 year old speakers die, the new 3.7s will definitely be on my list.