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

The question is did you listen to these before purchasing them, and, if you did, what kind of electronics were used.  Also, what kind of speakers did you have before?  My take is that the somewhat soft PrimaLuna, combined with the somewhat soft Maggies and a possibly unfriendly acoustic in your room is possibly the problem.

Maggies aren't for precision at all and by all possible means useless for studios. Their design is reflection of engineering, science and experimenting, but not accuracy by very far.

Those fuses you looked at.. Replace them.. I don't care what they look like.. -9db? That is a whole lot.. Any way to make sure the highs are actually getting an AC signal? or some serious comb filtering going on.. with that Di Pole

Is it down -9 with just one speaker playing? Turn off one speaker and run your test. Then move the speaker further into the room. One speaker.

The jumpers. Put the stockers back in and then replace the fuses.

Break in is not 5 or 6 days, it takes that long for cables to settle.
Get them off the floor too. If they are on the floor.. 3-4 inches is better. Just to make sure..

2-3 weeks and let the crossovers break in.

All I got for no money! :-)

It takes 4-6 months to break in a set of Teflon caps.. They sound terrible the whole time, too.

Patience and due diligence, make sure you're staying out of trouble instead of getting out of trouble.. You'll get it... I'm sure..

Regards

This may seem like to much common audio sense but maybe the room &  your equipment are a bad match: If you have another room , you can try an experiment to see if another room yields the same results: If you bought them new, I am sure the authorised dealer would be happy to help. Lastly, there is Magnepan customer support: I am sure you are not the only 1 to have these issues and they would more than happy to  help you.