I'm in the planning process for replacing all of my batteries aboard. You seem very well informed in all areas you comment on. I don't know your background, and please don't be offended, but I thought it worthwhile to ask what type of charger you have, what type of regulator on your alternator, and any "alternative" energy devices you have feeding your batteries?
AGM's are very sensitive to the proper charging voltages. In my case, the previous owner installed a high quality ferro resonant charger that is not suited to AGM's. My batteries are 5 yrs old and absolute junk. Before I replace them, i'm redoing the entire charging system. The result of incorrect charging is that they are always under charged and hence sulfated. Ferro resonant chargers (at least the one on my boat) cut in at 12.8 volts and cut out at 13.9v.
Everything I've read says the lifelines are great batteries and should last for 10-15 years if properly charged. Is it possible your alternator and/or charger is not calibrated correctly and created your battery situation?
For the benefit of those considering AGM batteries, the proper voltage for AGM charging is as follows:
Absorption 14.1 ->14.2
Float 13.1 ->13.3
I'd like to know what you have for a charger/alternator setup so I can avoid the $2000 investment I'm about to make in lifeline batteries if you've got the right gear and the battieries still fail early.
1998 HC 48T