Well ventilated area's will help keep them cooler, and running more efficiently, so under the seat, for an amp that runs hot.. isnt the best idea.
Do not attach it to the back of hte box, as over time, this will vibrate internal componets to the point of coming loose, or sustaining damage.
Out of sight - is always better, of course. Also - remember, the further away from the battery, or power source (alternator etc) it is, the larger the wire you will need, to provide the amp with adequate amounts of power with the least possible amount of resistance..
It's more efficient (and cheaper) to use a larger gauge speaker wire (which probably wont even be required, beyond 12g) and run it slightly longer, to your subs - and have your amp closer to your battery/alternator with the same gauge (but shorter) power wire.
I've never seen a problem putting any amp under the seat (unless of course the seat bottom rubs on the amp)....I for one use my AC all the time when it's even a little warm outside so the inside of the vehicle always stays cool enough for a properly installed amp to not overheat....I would personally worry more about heat if my amp was installed outside of the passenger compartment...just my $.02