I am considering installing an Infinity Basslink X powered sub in my 2003 EB Expedition.

I am wondering if anyone else has installed one of these, and what the sound quality is like.

And also, if the power and speakers leads from the factory powered sub can be disconnected, and utilized for this purpose. Or is it recommended to leave the existing powered sub, and run everything new to the head unit.

 

Have you looked at JL Audio for a stealth box? The bass link isn't bad (kinda pricey tho) but remember you will need to find a spot for it.
The stock sub amp's power wire wont be able to power something after market. Run your own wires.