Ms basspro-SQ vs 12 inch subwoofers?

Hello everyone I am in desperate need of help regarding my trucks audio system. I drive a 2002 ford f150 supercrew cab and i do not have a sub installed. I have a new stereo installed that is a kenwood touch screen and claims to push 50 watts x 4. I also have what i believe are kenwod excelon 6x8 front and rear door speakers.

I would really like to add a sub woofer to my set up but I can't fit a big box under my seats or behind them. I went to radio shack to ask around and they suggested a JBL Ms basspro-SQ which is a 300w enclosed 10 in sub. It has the amplifier built in so i guess its easier to install but it is $499.

I am worried that i will spend all this money and get stuck with weak bass. Really what i want is to be able to feel my bass in a similar way as i would a pair of 12's under my seat. Can anyone with knowledge of this Jbl sub give me advice

Any help is appreciated

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