Basement acoustics query

So, I'm mid-way thru building my basement studio and I have an acoustics question on the results i heard when i plugged in my amp. The room is 12' x 20' with a concrete floor (for now), painted drywall walls and a rigid insulation ceiling. The low end sounds awesome, but the high strings on a guitar sound shrill and piercing. Is this what slat (slot?) resonators kill?

Within the accuracy of you question, yes it helps that a lot. It is not he design intent, meaning thise frequencies, but yes, they help. Andre