Most significant upgrade

If you ask me I would get that Rega toy out of the system and bring Nottingham Spacedeck/Spacearm in.

The Rega P5 although a budget deck is quite decent. The P1 and P2 are the ones that can be classified as a toy IMO.

New batts in the hearing aid!

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.