I too am in the process of installing a stereo system into my 914. "Base Tubes" can sound good, but you can't integrate them into the styling of the interior. There are a few companies who make 10" and 12" sub woofers that need only .3 to .5 cubic feet of air. The sub I am going to buy is made by Alumapro (www.alumapro.com). After buying the woofer, I am going to fabricate a center consol what will double as a woofer enclosure.
I am using a set of kick panels that Dug Wright handcrafted that mount to using the stock speaker panel screw holes. They allow me to use 6-1/2" mid's without cutting up the door panel. Plus, the stereo imaging is great from the stock speaker locations.
I'll be keeping a photo journal of my progress once I start at
www.roadglue.com.
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Randy Wentzel