Just for the sake of clarity on this issue, the Dyna is a better match on the JMW 10 than the Shelter, so I think that needs to be considered when reading the above posts.
I do agree that the Shelter 501 MkII has a "lush" midrange, and seems to have a dose of the "Koetsu magic" in it. However, I would not characterize it as "slow" or "warm", except during the initial break-in period of the first 20 hours, when it seemed actually bloated in my system. After those first 20 hours, it tightened right up and sounded superb.
Lower compliance cartridges need a bit more break-in time than higher compliance ones, so that needs to be taken into account.
The Spacearm has been shown to be a workable match with the Shelter cartridges, as it has some stabilizing features that help with handling the low compliance cartridges. The later 10.5 series JMW arms also added stabilizers in order to make it match better with lower compliance cartridges.
As with any analog system, the match of arm and cartridge will significantly affect the overall sonic results. Getting a good match will always yield a better result, so it would be recommended to pursue an arm/cartridge combo that matches well. Not all arms match with all cartridges, and it is not a slam on any arm or cartridge if the match doesn't work out. That is the responsiblity of the user to ensure the matchup compatibility.
I like the Dyna 10x4 and 17D2 Karat quite well, but I now own the Shelter 501 MkII, and prefer that sound personally. I use a Teres 245 DC Signature 1.5 turntable, a modded OL Silver Mk1 tonearm, and a Shelter 501 MkII, and am quite pleased with the sound on all types of music. I have not used the Shelter on a Spacearm personally, but I have conversed with those who have, and they have reported good results with it.