I have a XS-D5100 up front and 3 Deka 9a31s in the trunk. Is this bad to mix and match? Will the XS drain the dekas?

No problem, IMO

more than likely no because the front battery rarely ever gets used for the car audio equipment in the rear when at least one or more batts in the rear are used.

Should be fine.

O.K. thanks. I'm taking these out of my car as I'm putting a new set-up in my truck. So I want to transfer the batteries to my truck. Which series should be in the engine bay and which one is the cab. As far as 3100 and 3400.

put the smallest one under the hood.

put the smallest one under the hood.

X2 I would put the 3400 under the hood.

Sorry to stir the pot. When I upgraded my amp I was looking into a group 31 deka for my trunk and someone asked what I currently have. (S3400 front, and D1200 in trunk) Person on other board said to just switch batteries like your saying shizzon. I called XS and they told me the way they are is fine and that I wouldn't notice a difference switching them around. Can you explain why you think theres a benefit aside from the only thing I can see of the bigger battery having more "reserve"

Sorry to stir the pot. When I upgraded my amp I was looking into a group 31 deka for my trunk and someone asked what I currently have. (S3400 front, and D1200 in trunk) Person on other board said to just switch batteries like your saying shizzon. I called XS and they told me the way they are is fine and that I wouldn't notice a difference switching them around. Can you explain why you think theres a benefit aside from the only thing I can see of the bigger battery having more "reserve"

The batteries are connected, electrically it shouldn't make a bit of difference.

Unless the batteries are isolated, I don't see how the front battery "wont be used by the audio equipment". That just doesn't make sense.

ordered my d3400 yesterday, should be here today.

Nathan, could you chime in on this. How would 2 Deka 9a31s compare to 1 XS D3100? I heard the D3100 will stand up to both Dekas combined. Can you comment on this? I'm sure the OP would like to know as well, could save weight or give a big boost in power.