ronkent -
You are addressing a major stumbling block of all audio reproduction. Most congestion / hardness that we perceive is heard in the high end. Most deficiency we hear is in the low end. Of interest is that our aural neurology constructs our heard recognition from the bass up. An effective default is to provide more bass and less treble - and it works because our ear-brain is such a masterful reconstructor.
Over these past years, I have been wrestling with these mysteries (at glacial speed.) My various solutions serve to add cohesiveness to the bass and subtract distortions from the treble, in simple terms. But changes are not in the frequency domain; before and after overlays are identical. The work centers on keeping the signal sorted out, which is already a strength of Thiel speakers. We are working in the realm of waveform flow integrity: in electrical, magnetic and physical propagation. Solutions are applied through geometric, dielectric, and fluid dynamic advances. There is significant chaos between signal arrival at the input terminals and moving air at the listener's body (not just the ears.)
What you are accomplishing by augmenting the lows by reducing the highs serves this perceptual purpose, but doesn't address the causes of the non-linearities you wish to mitigate. My solutions seek to accomplish what you want while keeping the frequency balance at its original design. One of these days first goods will come trickling out. They won't be electrical, but mechanical: relating to taming interfaces between room / floor, and driver / air, followed by field management in the speaker.
If you're up for more fun with your 3.7s, I propose a project. In one of your speakers, re-address your resistor pairs. With no other changes, reverse the axial direction of one of the resistors in each pair. Flop them end-for-end. You'll get the same circuit and layout geometry with same desired frequency balance.
If you choose to accept the project, let us know what you hear and we can discuss causes here. You may also PM me as you go.