I think amps are specifically never meant to be Upside down. Get creative. I just found a lot of room in the dash of my V, and I'm gonna put some kinda amplifier there, even though I don't need another one...

I like the idea of mounting it flush in the trunk.
before anyone suggests that i am agreeing with him cause he likes my idea of the flush in the trunk,

Serj is correct about amps not being ment to be mounted upside down. there are some that the heatsinks are designed for mounting in either way, but most are not. and even the ones that are have physics to compete with cause even in the element heat rises

this could lead to premature amp failure due to components that dont normally have large amounts of heat to deal with getting WAY too hot. i like the few on here that people have mounted under the seats as far as looks and i would make it so the seats were still removable, but it seems to be a wasted effort with the problems you most likely would encounter. the only way i would do it is if you have an amp that has fans on the sides and you could mount it so the heatsink was facing up when the seats were down, but then you would still have to space it away from the seat bottom so that the heatsink could cool down and not overheat in the carpeted underside of the seat.
lots of things to consider in addition to the cosmetics of amp placement.