I have Optima Red Tops in the Cobra and a few cars. The Cobra has two of the 6v wired together. The AGM batteries are a little tricky to "rejuvinate" when they go down.
I looked at a lot of chargers and ended up with this one from Griot's. It has worked very well and seems pretty smart. Has ability for you to select between AGM, Gel, and Standard lead batteries and also between 6v and 12v. It will go into "Restore Mode" on deeply discharged batteries including AGM's.
So far, it has worked on several different dead batteries of all types. I just connect it, select the battery type and voltage and leave it overnight.
Here is the link. It looks like now it has a big brother model too
Battery Manager V - Battery Care - Car Maintenance - Griot's Garage