Upgrade from martin 40i

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!!!