Hi Bob-
There is just something about the overall tone and natural rendition of detail of the 20X that really appeals to me. It is rich and warm with good weight. Overall sound is relaxed and musical. Maybe not the transparency and resolution of most MC, but it also doesn't have the glare or etch either. It renders detail in a natural and unhyped way such that I can listen to it forever. It simply manages to pull me into the music better than the others. What I hear with it is a great balance of tone, harmonic richness, detail, weight, coherency and musical flow. Nice lucidity in the midrange. Of those I have tried, the 20X has the best bass weight and slam.
I would put it succinctly like this: the 20X does not do any one thing exceptionally well, but it does everything very well. The Shelter is very sophisticated with a detailed yet smooth presentation. To my ears the 20X is meatier and has better tone. I have read that the 501 is really good with arms on the heavy side, and needs an arm heavier than mine (12g) to really excel.
The Nagaoka MP-50 is actually very good. Similar in character to the 20X in that it is warm, musical, weighty and un-hyped. My friend is selling one on Audiocircle now. It is the exact one I borrowed. Brand new stylus that I checked with my 40X lens. It is perfect. I think he might be flexible on the price.
John
PS: most of the carts I have tried are borrowed from local buddies. I own the 20X and Virtuoso Wood, and this experience tells me that I have chosen wisely. For the price, and even more, you can't go wrong with these two.