I have been slumming around the internet and stumbled on what might be a good upgrade to the stock sub in the Kappa. It is a JL Audio always good stuff it is roughly $129 and it with some effort most likely fit in the factory spot. It's the 8W3 V2 D2 - JL Audio 8"

It is not a shallow sub but mounting depth is only 4 1/2" with the large motor on the back it will be hard to fit, but if you put a 3/4" particle board ring under the mounting lip of the speaker, between it and the factory sub box, with a bit of trimming I bet it would fit. The rings are cheap on eBay, about $15 for 2. This would be a nice upgrade and keep the factory look. Just an FYI, never tried to fit one. (Rogue where are you)
The potent W3v2 subwoofers offer phenomenal performance at moderate power levels, making them ideal for those seeking sound quality and solid impact from compact enclosures.
These advanced subwoofers incorporate our patented Elevated Frame-Cooling technology for enhanced thermal dissipation and 20% more power handling than the original 10W3, 12W3 and 15W3. Cosmetic upgrades include a powder-coated silver frame finish, a dished cone/dust-cap design and a rubber mounting flange gasket that accepts grille mesh inserts directly (these grille inserts are sold separately).
Long excursion capability and highly optimized motor/suspension systems ensure clean, articulate bass reproduction with impressive low frequency extension.
8W3v2 DRIVER SPECIFICATIONS
8W3v2-D2 8W3v2-D4 8W3v2-D6
Free Air Resonance (Fs): 33.1 Hz 34.5 Hz 34.0 Hz
Electrical "Q" (Qes): 0.392 0.468 0.465
Mechanical "Q" (Qms): 7.84 7.84 7.84
Total Speaker "Q" (Qts): 0.373 0.441 0.439
Equivalent Compliance (Vas): 0.79 cu. ft. / 22.4 liters 0.79 cu. ft. / 22.4 liters 0.79 cu. ft. / 22.4 liters
One-Way, Linear Excursion (Xmax)*: 0.39 in. / 9.9 mm 0.39 in. / 9.9 mm 0.42 in. / 10.7 mm
Reference Efficiency (no): 0.199% 0.189% 0.182%
Efficiency (1W/1m)**: 85.2 dB SPL 85.0 dB SPL 84.8 dB SPL
Effective Piston Area (Sd): 33.2 sq. in. / 0.0214 sq. m. 33.2 sq. in. / 0.0214 sq. m. 33.2 sq. in. / 0.0214 sq. m.
DC Resistance (Re): 3.35 ohm*** 6.70 ohm*** 10.31 ohm***
Nominal Impedance (Znom): Dual 2 ohm Dual 4 ohm Dual 6 ohm
Thermal Power Handling (Pt): 125W 125W 125W
Driver Displacement: 0.035 cu. ft. / 0.99 liters 0.035 cu. ft. / 0.99 liters 0.035 cu. ft. / 0.99 liters
Net Weight: 7.0 lbs. / 3.2 kg 7.0 lbs. / 3.2 kg 7.0 lbs. / 3.2 kg