How about this sub/woofer for the underseats:
Dayton Audio NS210-44 8" Neodymium Low-Profile DVC Subwoofer? Looks like it'd be an easier fit than the Morels. If I blow a Jehnert I might try one. Hell, at $99 I might try fitting one for kicks and grins.
Specifications: • Power handling: 200 watts RMS/400 watts max • VCdia: 2" • Le: 0.85 mH • Impedance: 2 ohms • Re: 1.82 ohms • Frequency range: 35-500 Hz • Fs: 38 Hz • SPL: 91 dB 2.83V/1m • Vas: 0.51 cu. ft. • Qms: 5.74 • Qes: 0.39 • Qts: 0.37 • Xmax: 7 mm • Dimensions: Overall diameter: 8-1/4", Cutout diameter: 7-3/8", Depth: 2-1/2"