I've got 3 stacks (John Risch cheap 'n dirty bass traps) of R-13 fiberglass that comes in rolls in corners.
That tamed the bass boom in my room: poorly suspended wood floor.

I cut plywood circles for the tops of the stacks and stapled colored burlap to the edges which skirt the rolls down to the floor. Kat likes to sit on the one by the window. After she bakes you a pie you might let the little woman choose the fabric herself.

The thicker the insulation the lower the frequencies absorbed so
your R-13 shouldn't be lightening the treble. Just the opposite. There should be more clarity. I think you're hearing the lesser extension of a horn tweeter compared to your Thiels. Might want to consider a JBL 2405 or other super-tweeter.

Just my buck and a quarter.