I have the MKIV-12. It has to be the most powerfull sub I have ever heard. I was going to purchase one of the two grand REL models or maybe a Sunfire until I heard this beast.
Granted, I use this sub only for HT so I can't provide an opinion as to it's musical abilities. Also, I understand that the 10" models are built in China, and are not nearly what their big brothers are.