My New Magnepan 1.7i's sound dull and lifeless on the top end...why?

Not to be argumentative, but Maggies are NOT ribbon loudspeakers. Well, the tweeter in the 3.7i, 20.7i, and 30.7 is a ribbon driver, but all the rest of the Maggie line and all their drivers are planar-magnetics, not ribbons. A "ribbon" driver is a very specific thing (the diaphragm of the driver and the conductor that causes it to move are one in the same), not just any ol’ planar.

Calling Maggies a ribbon loudspeaker is like calling an electric bass a bass "guitar". Common (Steve Guttenberg regularly refers to Maggies as ribbons, and John Atkinson always calls his electric bass a bass guitar), and incorrect. ;-)

@bdp24 your point is well taken. The point I was trying to get across is that Maggies do not move air. They don’t have traditional drivers they move air. Hence, no persuasiveness in the sound. 
 

@charliech No high pass filter used. RELs were connected to amps with high level REL cables made by Kimber and later Analysis Plus to amp binding posts. Crossover was tuned to my Maggies and the room. It’s been a while since I had the Maggies in my system. But they were crossed likely at around 80-100Hz. 
 

I would also recommend getting the Mike Powell silver Ag Maggies upgrade and putting the 1.7i pairs on better stands. 

One difference between open baffle dipoles (such as planars) and dynamic drivers in enclosures (whether sealed or ported) is that dipoles "move" air from one side of the planar to the other---leaving the air pressure inside the room unaffected, while box speakers change the air pressure inside the room (and inside the enclosure, which is why the suspensions of drivers intended for use in enclosures differs from that intended for open baffle use).

@kingbarbuda, did you mean to say "percussiveness" rather than "persuasiveness"? If so, I find the bass panels of the big Maggies to have a very percussive sound, which is why they reproduce piano and drums so well. But that’s more a mid-bass rather than deep-bass (40Hz and down) phenomenon. Magnepan has been working on a dipole woofer system for a few years now, which uses multiple dynamic drivers in a sealed enclosure. It will be available as an add-on for current Magnepan models, and as the woofer system for a new model: the 30.7 For Condos.

Eminent Technology already offers such a woofer system, and then there is the state-of-the-art OB/Dipole Sub from Rythmik Audio/GR Research.

I sold off my 1.7i’s not long after I had them solely because their height and overall presence in my room made me feel as though I was on Easter Island…

I replaced the 1.7i’s with a pair of .7’s, which I found to be much more responsive, perhaps even “brighter”, for lack of a better description. I had a Pass Labs X150.5 during this period, as well as a couple of tube preamps - all was good. 
Fast forward to the present and I’m now absolutely digging a pair of original MMG’s whilst I await the arrival of the new LRS+.  Pass with Maggies and Thiels is usually a solid matchup. 
 

My current setup is a good deal less involved with equipment. I’m enjoying a Belles 250i hybrid Mosfet integrated amplifier with the MMG’s, a coupla Schiit dacs, a cd transport, a cheap-seats pro-Ject turntable, and an Aurender 2TB music server. Back to basics more or less and everything is fine. Oh, and no sub. 

@bdp24 Yes. I meant “pecussiveness”. Autocorrect messing me up again.

My experience with Magnepans are my former 1.7i and the AXPONA tests of the LRS and the LRS+. I added a pair of REL T/9i subs to my Maggies and it made a world of difference. Not hard to dial in. But I think the addition of the subs and what kind of subs could be room dependent. I have thought about getting a pair of LRS+ just for kicks. I haven’t pulled the trigger on that yet though. Going to AXPONA at the end of the week. Maybe something will happen there.