Local audio guru sez a plate amp is needed and that it will
give a useable volume level, unlike patching through the
Radio Shack. But I'm wondering what the difference is (?)

Looking up the cheapie plate amp at PE....

Well it gives some detail but not what I was looking for.

About the most efficient entry level product I can gather
is one of these and the MCM 6 in subwoofer driver w/16 mm
Xmax as shown with the Tuba 18.

Interestingly, the tubas are getting a bit of discussion
over at the Full Range Driver Forum.

• PE compact subwoofer