Most significant upgrade

I'm not a fan of your speakers so my suggestion would be with the Vandersteen 2CE sigs mentioned above. If you are set with the MLs, I would suggest a matching sub.
Next on the list is room treatments.

This probably isn't the type of answer you are looking for but I have to ask do you have dedicated 20 amp circuits powering your system and what type of AC outlets are you using?
I ask this because some audiophiles would be shocked at the difference in sound when they use dedicated circuits and quality outlets.
Just a thought.
Nice system by the way.

Just kiddn.. But they do help. This is tuff with out knowing your system I could not tell you what to upgrade. you upgrade your weakest link for what you listen to.

How much are you willing to spend?

I have been upgrading for the past two years. My experience upgrading requires that I provide you two answers. I submit speakers are the foundation of a system and you should build around them. I just added an Audio Research DAC7. I don't want to exaggerate, but it sounds like I have new speakers. The sound-stage has opened up and music is much more enjoyable. I don't know if the Rega Apollo is that bad or the Audio Research is that good. Adding the DAC has had the most significant impact on improving the sound "IN MY SYSTEM."