Nick
I would echo what George suggests about setting the front 3 by D5200's to Large.
As an addition, I would test eliminating a Subwoofer from CD Replay. This is easily done on the Meridian Configuration program by Setting Sub to "None" for Music and then to e.g. "LFE" for Movies only.
Personally, I always find a better balanced, more accurate and pleasing sound will be produced by using Full Range floor standers, which is what you have. Integrating a sub is difficult at best, and rarely does a sub exhibit the dynamics and response that a good floor stander like the D5200 does.
One other thing to try with Trifield is to change the "Depth" setting. I note that you have it set to Zero (I think you said). Pretty sure the default is +1. This default setting almost works counter intuitively in that a setting of +1 pushes the sound from the centre back, thereby increasing the perceived depth. Setting it a Zero has the effect of bringing the centre speaker "forward" which is possibly not a good idea (depending on the room and speaker set up)
Anyway, give it a try and make up your own mind