SS round core vs NPS round core test in progress:
Fender J: E,D Hi-Beams, A,G Sunbeams
J-type: E,D Sunbeams, A,G Hi-Beams
Results (not proof, but enough care taken so I can rest with the conclusion):
1) I don't think new Hi-Beams are brighter than new Sunbeams (which reminds me of the exaggerated reputation for brightness of new Roto 66). I think new Hi-Beams and Roto 66 are very crisp, and crispness implies some highs, but I don't think they're brighter.
2) As they break in, the high rolloff of Hi-Beams (and the other SS rounds I'm trying) is natural sounding, like turning down a tone/treble knob, leaving an impression of good mid-focus.
3) But new Sunbeams - and this persists as they break in - in comparison, have a barely noticeable "quack" (I have to turn the tone knob all the way up, which I almost never do, to detect this, so it's no big deal, and I noticed it with Slinkys too). My personal theory I'll go with is that, unlike the smooth rolloff from turning down a tone/treble knob (like Hi-Beams), the NPS string has some damping of highs (as we talked about above), but there remains an artifact - as though the string is audibly a composite structure (which it is of course, as is the SS string, but I'm saying there's a discontinuity in the damping) - that leaves whatever freqs that cause a quack sticking up a little. Barely noticeable.
EDIT: Some people might hear that difference as a bit of extra excitement in Sunbeams, and they might hear Hi-Beams as more plain sounding. I could see that.
I'd say for now that this tiny difference is worth the slightly rougher surface, to me. So when I do round core vs hex core both will be SS.
Of course I could be deaf so don't take it too seriously.