Flat Wall Mount Stereo MLTL using TC9FD's (2)

Hi, my first time building a speaker, learning from my mistakes, but after getting it kind of done, I found that I must have done something wrong, as I'm failing to make the treble (I guess above 5KHz) sound acceptable. Of course, I don't expect it to sound perfect, but what I'm getting isn't even mediocre.
I suspect it's because my I'm mounting the woofer to the inner side of cabinet (as it just looks so much better when the square frame isn't visible from the outside). Is it possible that that's what's eating the treble? I feel like the more I chamfer the edges of the hole (manually using a rasp&file&sandpaper) the better it sounded (until I accidentally destroyed one baffle in the process by making the hole too big for the woofer / impossible to assemble it air-tight)
Or what other noob mistake could be eating the treble?
I used 2 layers of visaton polyester for dampening.
Another thing I did bit differently than others is that the woofer isn't screwed only into the baffle, but also into M4 standoff metal (but not magnetic) spacers behind the baffle, which are mounted into the back side (like seen on the attached photo), making it more rigid.
It looked so nice already at one point, but sounded so bad after all the work and many attempts... not giving up yet, but if i will, it could still make a nice birdhouse :)
 

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Thanks, it was perhaps that too, but I also found out that the issue was totally elsewhere - the amp I was testing it with was dying and distorting... after making the holes just slightly wider near the rubber surround and switching to another amp, it sounds good. Actually quite impressive. Didn't expect that a single 20eur driver could sound this good. Will post photos when after finishing...

By the way, how do I simulate the wall proximity / boundary effect of this kind of wallhanging speaker? I'm just learning/playing with the VituixCad and SpeakerSim software, but it doesn't seem to care about the wall at all. Is that a part of the baffle step simulation or where do I look for it?