I've been a huge fan of Green Mountain audio speakers up until about two years ago and had various models including the Europas, Callistos, EOS HX and Continuum 1.5s. I've heard the C2s (very similar/same as C3s) on many occasions as well but actually preferred the 1.5s as the bass was tighter but the C2/C3s much more efficient. I thought the C2-C3 were excellent from 90 hzs on up but the bass a was a little loose below that in my listening sessions at my friends house with a few different amps. It always felt like there was a bit of a hole or discontinuity in the upper bass range where the 12 inch woofer handed off to the five inch mid but that was just my impression. I didn't hear the HX crossover upgrade.
I do agree the screws would stop inserting in the stone and I also had to apply putty to the underside of the baskets in the monitors to stop the ringing and it did work to prevent that but was odd that Roy did not add putty or correct that when assembling them.
Anyway what I admired about those speakers was the open immersive sound all The Green mountains projected and lack of boxiness so was a big fan for about 20 years. They truly disappeared and were very revealing of any component changes. You had to be careful in the amplification/DAC, etc. used as the sound could become a little harsh but when you had the right amp/DAC they were phenomena. Still to this day the C-2s/C1.5s are one of my favorite speakers I've ever heard.
I've moved onto the Spatial audio X5's open baffle speakers and they are just better in pretty much every way. The Spatials have that same open three dimensional midrange and excellent depth but the treble is even more satisfying and the biggest change is the bass to upper bass region where there is absolutely no contest. Dynamics and bass instruments are much much better and clearer. Obviously having no box gives that very open sound as well that even the Green Mountains cannot match.
This was the one speaker I heard that I finally said it is better than the Green Mountain Audios. I still have a pair of the Callistos in my home theater setup.