Choosing Amps?

Like I said, Hifonics generally makes decent gear but I do not know this particular amp.  Buy whichever one you like the most and can afford.

Under the rear deck?  You mean under the parcel shelf?  Just don't mount it upside down.

Because all the electricity will fall out.  Just kidding.    The truth is that amps will not cool properly and can burn up (indeed, can start fires) or at the very least have early failure due to overheating if mounted upside down.  Almost any owner's manual will tell you this, and it is important.

Only the smartest ones do. 

Yes, or build a shelf below the rear deck to mount it to.  You may want to add a fan for any high-power amp.

Where is that smart arse icon when you need it, "the electricity will fall out", I have not heard that one in years.... If you still have the link to that amp, send it to me please, I might just need to buy that Eclipse amp.