Both the exciter and panel material you have chosen have problems with HF ,over on DIY audio ,a study of dml ,some report a roll off as low as 3k!!! And use EQ to "fix" the problem?
The exciter should be properly glued to the panel ,you could also thin the panel in front of the exciter area to allow more sound to emit from the central area.
XPS heavily damps the sound, EPS ,even at 1inch,in my opinion is far better if used properly.
Hope this is of some help.