I chose to go the path of MLV + CCF. Rather than lay them down separately, I bought a roll with them bonded together:
Privacy Performance Carpet Underlay
I bought it from www.jss.net
, total cost was just shy of $300 with shipping.
I started the roll from the back of my EB van and ran it continuously up to the front footwells as one continuous piece. It fits perfectly between the wheel wells, and there's enough left over to go back and fill in the spaces on the side in front/back of the wheel wells, in the steps, etc.
I bought a datalogging sound meter so I could quantify the impact on cabin noise (what can I say, I'm an engineer), but unfortunately the timing worked out such that I didn't have a chance to do a 'before' measurement. Right now I have the PPCU all rolled out of the way for L-track installation, and I plan to take measurements before putting things back in piece by piece (including Noico 80mil butyl) so I can quantify the difference.
The l-track floor project has been a long time coming and the van was a work van with trunk liner sprayed on the floor but no other flooring when purchased, so we've been using the naked PPCU as our floor for over a year. It works surprisingly well, nice and soft underfoot, but the MLV top layer can gouge or get marked with white 'stretch' marks relatively easily.