According to HSU, I believe the MK3 will have a little more power in the 40hz and above range if I recall correctly and the HO will go louder and deeper below 25 hz.
The MK3 is definately the better value right now at 200 less.
I have the original radio Id like to install, but I want to test it first. Its model #986846, AM only. There are 3 male contacts on the back. What is each one for?