Most significant upgrade

I think Mezmo is right, your system looks pretty good as is . To get a real "upgrade" you are going to have to spend a bit of coin. The one thing that may not cost you much but give some good results would be to sell the speakers and get a pair of Vandersteen 2CE signatures in that system.

The reason I suggested the 2CE sigs is because I am familiar with the sound of both your current speakers and the 2 CE sigs. For the type of music you listen to, and your integrated that you are using. The Vandersteen speakers have a frequency range of 29Hz to 29kHz, 6ohms nominal 4 ohm minimum. Your MLs are 40Hz to 20kHz, and drop down below 2 ohms in the upper frequency extremes, the Harbeths are 46Hz to 20kHz but I am not familiar with the sound of Harbeth speakers so I will not comment on that. So the Vandys will play lower than the other two. I also agree completely with the room treatment suggestions as well. BTW your Rega is not a toy.

Dan