My used Magnepan 1.6's are missing sparkly treble. Why?

I thought I should provide an update to my original post.  Since I wasn't sure what was going on with the 1.6's and there was a very good used 1.7 for sale locally at a good price I ended up getting that also.  Overall I liked the 1.7 better and the treble might have been a bit better but still not what I expected.  However, I kept playing around with the placement of the speaker and positioning the 1.7's tweeter on the inside and a little more toed in and that definitely improved the over all sound but cymbals and top hats still seemed to be somewhat lacking.  Reading more posts about how others had placed their Maggies I noticed many users thought they were better tilted forward slightly.  So today I made that small adjustment by sliding a CD case just under the rear stands.  And that definitely made a big difference in the overall sound and especially the high end.  The 1.7's now sound realistically bright and closely match what I'm hearing if I listen on the Ananda headphones.  And that also may explain why the 1.6's seemed so disappointing since they were actually set up to be tilted back by about the same amount as my 1.7's are now tilted forward.  I feel like I mostly have the 1.7's dialed in now but it is amazing how sensitive they are to small changes in positioning.  That fussiness is probably why they aren't more popular.

Tilted back?
Oh heck no...lol
My baby Maggies gave me a huge improvement in SQ when i played with positioning but mostly due to me tilting them forward.
And yes tweeters on the inside gave best results.
There likely then is not much wrong with the 1.6 to start with

I think you are probably right about the 1.6's being fine although I'm probably not going to go to the trouble of hooking them up again to find out.  I'll put them up for sale soon and hopefully somebody else will fall in love with them.  At the used prices these Maggies are one of the great values in the HiFi world.

I totally agree with you OP on vfm.
My pair of $500 Maggies bested an $8500 pair of Josephs in my room with my gear.
Thats obviously not implying they were better, just  a better fit with my gear, room and my ears.

But my present Spatial Audio ob speakers are another huge step forward over the Maggies for me.

The Spatial Audio speakers look very interested.  I think if a used pair of M3's or X5's came up I would probably give them a try.