Xtian,
My system doesnt have the weight and body..
The last post immediately stuck to me that you are missing the correct "gain" in your system. Then I checked the spec on the ProAc and find that they slightly less efficient speakers. I have had similar experience when I moved to a larger volume room. After I got the correct gain, my speakers now sound awesome.
The Rega is 50Watts, while the Leben is 15W. I think, for the ProAcs to sound better, you might need more power. So going to Leben may not help. The Harbeth is less efficient. Now understand that I am basing this all on the numbers I am seeing on the component sites.
Do you have anybody close by that you can reach out to, who has a more powerful amp? You might want to try that amp in your system and see if you get back what you are missing. That might be your solution.