Buck 5,000+ posts
little alien on campus
Both subs receive the same amount of power if wired correctly. It is possible for one sub to be driven harder if any of the voice coils' impedance is out of spec.
I wondered this by the way that read to me.
OP, have you checked the ohm load of each sub individually after they're wired in series? You may check and see that and maybe even read each voice coil independently on each sub to make sure you don't have a bad coil.
Not saying this is the issue, but it sounds similar to a different issue I had seen someone have a while back...
The ports this guy had on this box where..... unique.... but way, way wrong! I can't even explain how badly this enclosure was designed. He was having a problem with only the subs (I think he had 6 of I remember correctly) closest to the port heating up, not the others. The issue ended up being because the way these ports where set up, it was making those subs reach their mechanical limit before the other subs.
Like I said, I'm not saying this is your issue, and I wouldn't even know how to fix it if it is because I'm not a box designer by any stretch of the imagination, I just figured I'd throw an idea out there that maybe others weren't thinking of.
Is the sub that's getting hot the sub closest to the port?
This can be an issue. It usually takes a really bad design for this to happen. You could have one sub that's unloading way more than the other, or you could have a sub that's not moving properly compared to the other due to the box design.
OP, post up a pic of your box, if you want, and we can see if that could be a potential issue.
OP, do you monitor what voltage your amp is seeing when you're bumping hard? You may be getting low voltage to the amp, and that's usually the quickest way to burn a sub's voice coils up.