Most powerful amp my truck can handle?

Your current setup without a AGM battery you can Probably do about 1800 watts leaving the last 200 watts your alt can produce at its max capacity to charge the battery.
At idle you will see voltage drop with anything around 1400 - 1500 watts.

You will only see the true 1950 watts your alternator can do around 1000-1500 rpm.

Convert that 140 amps to watts via a 14v constant system, thats how you will get the actual limit and not guess.

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Thanks for the great reply. At idle it doesn't drop but then again it's only 1400w. So far never had any issues and that amp hasn't seen anything below 13.5v.

With one of those fancy D3400 XS power batteries what could I put out? Not sure how the calculations work with batteries. It would replace my starting battery (Current starter is good, so if better feel free to suggest a battery to go in the back of my truck by the new amp.

I'm getting an X12 subwoofer and am currently thinking of a PDX-M12 1200w amp ($500) but if I can find a better/cheaper/higher output amp I'll put the savings into a new battery if that makes sense.