Upgrade from martin 40i

Monitor Audio

Ascend towers. https://www.enjoythemusic.com/magazine/equipment/0312/sierra_tower_speaker.htm

Check out the KEF R11’s.

I know you're looking for new speakers, but here's some info if it's useful for you: I have ML 40i speakers and thought I had blown my tweeters. I called ML service and they sent new tweeters immediately, at no charge. It was an easy install, took about 20 minutes for both. Great service from ML. Best wishes!

+1 @phasemonger 

Even if you want to upgrade, fix them first.

I agree look into fixing your MLs, especially if you like them and if it’s that easy. That aside, what improvements would you be looking for with new speakers if you go that route?

If you live in the northeast just go pick these up if they’re still available.  I’ve heard your speakers and these will stomp your MLs into the ground on every level. 
https://www.usaudiomart.com/details/649878811-joseph-audio-rm25si-signature-speakers-with-750-upgrade/

@ramshackle The ML40i speaker is decent for Home Theater (HT) use. Worth fixing and replacing both of the Folded Motion® tweeters if they are affected. Call ML and ask about replacement. Best to check all connections at speaker and amp. It’s worth having the MRX 720 amp checked if you suspect a channel problem before playing the speakers again - - and before any tweeter replacement.

I have to say I’m very curious as to what the final result has been with your ML 40is.  
Did you get your new tweeter installed?  Did that solve your problem?  Thanks. 
 

Yes new tweeter really did help out the sound!!!