You got bad advice. C4 needs more space. You should not be closer than three feet to the back wall - very bad listening position. Diffusion will not help bass which will be bad near a wall.
You need something like ATC SCM 25 or 50 in a nearfield configuration. A speaker with critically damped bass will help. This would be a big step up from Dynaudio C4 which have muddy excessive bass but would retain or possibly improve upon the mid range and treble clarity of the C4.
You obviously have excellent hearing acuity and your comments about speakers are bang on accurate. I concur on Wilson - they get bashed here a lot but in general they have better mid range than many others. Unfortunately Wilson have resonant excessive bass and therefore this is not an option unless you can find a much bigger room.
You need something like ATC SCM 25 or 50 in a nearfield configuration. A speaker with critically damped bass will help. This would be a big step up from Dynaudio C4 which have muddy excessive bass but would retain or possibly improve upon the mid range and treble clarity of the C4.
You obviously have excellent hearing acuity and your comments about speakers are bang on accurate. I concur on Wilson - they get bashed here a lot but in general they have better mid range than many others. Unfortunately Wilson have resonant excessive bass and therefore this is not an option unless you can find a much bigger room.