I just installed these two amps under my seats tonight. The mono amp is slightly larger and had to go under the driver's seat. The space under the passenger seat is actually tighter and luckily I was able to to fit the smaller 4 channel amp under there. I am able to safely adjust the seats forward/backward without any snagging. They cannot be seen when the seats are installed. Using Amazon will help you save money. These amps are cheap and effective.
Amazon.com: MRP-M500 - Alpine Monoblock 500 Watt RMS Power Amplifier: Electronics
Amazon.com: MRX-F30 - Alpine 4-Channel 600 Watt Max Speaker Amplifier: Car Electronics