Everyone knows my love of the 103R.
My limited experience of one sample of a Shelter 501 (for a one month period) pointed me toward it liking rock and modern jazz but not having the coherence of either it's big brother (the 901) or the Denon when playing anything acoustic - be it bluegrass, chamber, or large scale symphonic music.
It's been my experience the Shelter line doesn't come into its own until you get to tht 901.
Please take this as one person's experience of a single sample of a 501 for only a month's time, and of and extensive sampling of 3 different 103R's.
As far as matching the relatively stiff 103R with a medium or low mass tonearm, I recently added about 5 grams (via a small brass shim) to the tonearm of a Dual 1228 for a visitor.
I was mightily impressed at the amount of music coming out of this rig. The turntables's owner wasn't as impressed as I was, but I am convinced that he was freaking out over my Exemplar horns. It was obvious to me that he was very disoriented, as he lives in the land of cones 'n domes.
Cheers,
Thom @ Galibier