I respect your opinion and I never mounted my same cartridge sample by B&O in the STAX but I did it in at least other 4-5 arms and always with a degraded sound quality level against the same cartridge mounted directly in a universal headshell.Hi Raul . . . I'll admit that I haven't tried the 1/2"-mount adapter with a conventional headshell to mount the B&O cartridges specifically on the UA-7 . . . quite frankly, I'm a bit short of headshells, but on your recommendation I might give it a try when I get the chance.
That plastic headshel type used with the 20CL is more or less the same concept for some of the Acutex models and I experienced the same disappointment with: sound degraded quality level.
I have found that it's quite the moving target to get B&O cartridges to perform their best . . . and will concede that I've seen quite a bit of variability in the manufacturing and condition of their adapters and "headshells" for non-B&O tonearms. I actually had to combine parts from three "integrated"-style adapters, carefully trim the molding flash, and use a stronger clamp fastener before I ended up with one where both parts fit well together to allow positive overhang and azimuth adjustment, as well as firm grip when the cartridge snapped into place. There's also the issue that the integrated-style headshell grounds the cartridge case to one channel of the cartridge leads, rather than to a separate pin as on the 1/2"-mount adapters . . . I can see this being an issue for some phono stage designs (notably some with balanced inputs) I've seen.
It does also seem that in general you've had better luck with lower-resonant-frequency cartridge-tonearm combinations than I have, and I can only speculate as to why this may be the case. IIRC the measured resonance peak I'm getting is about 9Hz with the integrated headshell, and since I've never had a good experience with any combination from The Land of Five Hertz . . . maybe the lightweight Ortofon wooden headshell will be a good match.