Shelter 501Mk2 or ZYX R100-02H

Not sure I can help with that specific comparison, but you might want to contact Mehran at Sorasound to get an idea of his impressions. dougdeacon here has also written a good deal about ZYX carts, you could look at some of his post too. Not sure if he listened to the R100.

Also, I get the impression that in general, the low output ZYX carts are more highly thought of than the high output.

Again, NO direct experience here, but these were impressions I picked up when considering the purchase of a 4D (which is freaking incredible!).

Yes, but more info even from an interested party is still a useful data point. The mistake is to not take into account just that party's opinion.

Not sure if Pani has heard the specific cart in question that you are asking about, but from reading XYX reviews it seems that they differ across the range with regard to the how live and revealing ia specific cart model is. For example, see dougdeacon's review of the 4D, where he compares it to the UNIverse and concluded that the 4D is a better rock cart.

Moerch recommends their heaviest armwand (the Blue, effective mass of 14g) with the ZYX carts.

As an aside, my do I love the Moerch's feature of swappable armwands of different mass (from 4-14g!).

If you've been really good, ask for a 4D. Love mine.