OP, why 6V??? I've gotten good service out of my 12V AGMs.
AGMs... my best mod until going residential. At three years from new I spent a day lowering my house battery rack so I would have room for a second rack, scrapping all the rust and repainting with acid resistance paint. Decided to go AGM and should have sealed the compartment as every couple years I take a hose and knock the dust off the top of the batteries. That was 10 years ago and I cannot tell a big difference in battery capacity although going totally LED has helped. I recently added four 120AH 12V AGMs for my residential refer (some on their sides) and paid $75 each (they were blims) from an Atlanta battery wholesaler.
PS: The AGMs were off brands made in Canada.
PSS: I got 10 years out of my Interstate engine batteries.