People mix batteries all the time because either 1, they dont know the risks of mixing batts. Or 2, they know the risks but do it anyway. The wet cell WILL shorten the life of that agm. If you want to have a battery at all for car audio I'd replace that wet cell with another agm or switch it out with that bt100 and rid that wet cell all together. That agm is much more expensive than that stock batt man. It's doing yourself a favor.