Krumbie: You can buy an amp that accepts Speaker Line Level Inputs, then all you need to do is use the signal from the Bose harness to convert to RCA inputs.
apeezy08: I originally thought of adding an additional Bose subwoofer under the passenger seat also. However the Bose subwoofer is a 1 Ohm sub, and there aren't any available 1 Ohm subs readily available. Plus, adding an additional Bose sub wired in Parallel would reduce the resistance down to 1/2 Ohm, or in Series would double the resistance to 2 Ohms. Either of which will either over work the factory amp or provide insufficient power to both Bose subs.
As far as aftermarket, there isn't much available that will fit under the seat. I opted to go with a Pioneer Shallow-Mount 10" sub with a custom built enclosure under the driver's seat, and an Alpine MRP-M500 under the passenger seat to drive the sub. I considered the Cadence unit also, but I'm not a fan of single unit powered subs due to the fact if the amp or sub blows, you would have to replace the whole unit. I prefer a better sound quality over the convenience of the single unit.
Just my 2¢.........
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2011 Armada Platinum in Nordic White
K&N Intake, 2° Advance, 6K HID Headlight & Foglight Conversion, LED Conversion, 24" Boss 330 Black Wheels with Polished Lip, Toyo Proxes ST II 305/35/24 Tires, 2 Pioneer TS-SW841D Shallow-Mount 8" Subwoofers in Custom Underseat Enclosures, Alpine MRP-M500 D Class Mono Subwoofer Amplifier, OEM Hood, Window, Sunroof Deflectors.