i dun see any difficulties to mount the 3 amp at the boot...do u?
install amp under seat is old way that ppl lazy/no budget to make a good amp mounting board at the back....
the bad thing is the dirt, vapor or even water will get into the amp easily....not to mention it had block the leg rest area under the seat for the rear passenger......especially kcar......
btw, u will definately need monoblock for ur sub to work well with less hassle.....class D circuit design are a very good design because it produce less heat, bigger power and more efficiency....and ur sub are double 4ohm each with 300rms each sub...not every 4ch amp can do tat job...and if can, the price for the amp are double the monoblock price.....while it will product higher heat, lower power and power hungry..........(less efficiency)