What to upgrade?

Looks like a good amp and nice speakers. You probably just need to add a little "muscle" in that larger sized loft to retain good dynamics at higher volume.

If sound is good but dynamics lacking at higher volume, I would consider adding a pair of good powered subs. That's the easiest way to just throw some more power and air pressure at the problem. Crossing over to subs for low bass will also take load of the monitors and allow them to do better as well at higher volumes.

Do you use any room treatments for the acoustics?
That could possibly be the culprit?
With that said I must admit that I'm not familiar with your current speakers or integrated amp. Can you tell me more about them?

No doubt room acoustics in a loft can be tricky. Might wreck havoc on soundstage and clarity with perceived dynamics more the victim.

DEfinitely always a good idea to assess and get a handle on teh room acoustics before making a major change. That's often the biggest determining factor regarding what will or will not work. Good sound from gear heard elsewhere may not matter much.

The sub idea might still be the effective boost needed. Placement options with separate subs to compensate for room acoustics might be easier than just replacing with all self contained larger speakers. Corssed over correctly, existing amp should be able to perform more optimally as well at higher volumes with low bass offloaded.

Take a look at some of the audiokinesis Planetarium speaker systems that leverage up to 4 separate subs to help compensate for room acoustics. This might be a fantastic approach to consider in your case. I have not heard these but love the innovative approach and thought processes that go into Duke's stuff to wean out performance for reasonable cost.