Remember a car sub sounds nice deep and loud in the extremely tiny confines of a car's interior. Put a sub outside the car and its not going to sound nearly as good.
(my car came with a factory powered subwoofer - a single 6" speaker as a sub under the passenger seat! And it does a surprisingly ok job at normal volumes - but its no match for a pair of 12's in a car. But a pair of 12's in a car box is no match for a 2x12 bass cabinet.)
You don't want to move the Henry - I understand, its not light (though if you have a hatch/wagon you just tip it over and it slides right in the car. Put a blanket over your bumper and load in/out is easy. Get a bandmate to help carry it up any steps or on to elevated stage).
A single 12"... you need a sub box with the right speaker. Don't think the Avatar is going to cut it for what you are asking - especially as you have blown 5 speakers trying. You could possibly find a 12" sub box - look at PA subs and check their specs. Maybe try one out at a store to see if it gives you the volume you want.
Maybe try the 12" as a low-mids cabinet. Set the crossover higher and add a hpf then add a powered sub to that. The 12" cab will give you the punch and the sub will cause the shaking, while your 4x8 takes care of the mids and highs.