Kevin,
Yeah, I took one apart and noticed that insert came out. I wasn't sure if it would create a leakage problem but didn't figure it would. These towers are going to have dual 10's in a sealed enclosure so I don't want any issues. I imagine the washer on the inside will help that quite a bit.
Yeah, I know exactly what you mean about the cup. Same concept as any other except you don't have to worry about quick disconnects or soldering. Just tighten the set screw.
I'll probably call you in the next week or so to place an order.