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!).

Thanks for the advice, but Sora Sound is interested party...
Opinion of Auduogoners is more interesting for me...
:-)))

On an Avid the ZYX will be too live and revealing for Rock. I would go for Shelter.