I called support, and the answer is NO, you cannot make a "Full Stack" with the Lin6 Spider 2 cabinet.

To make a "Full Stack", you would have to buy 2 other cabinets, then it would work as 2 x (4x12) cabinets, each cabinet would need to run at 8 Ohms MONO.

Here is why, and what options you have:

#1 - The Spider2 4x12 cabinet has 4 x 16 Ohms Celestion speakers and 2 jacks on the back.  If you use both jacks, you effectively spilt the cabinet into 2 groups of 2 speakers, wired in parallel to give you the 8 Ohms the amp wants.  If you use the Left jack only, it will run the entire cabinet in parallel and reduce the Ohms to 4, so if you have an amp that can drive a 4 Ohm mono cabinet, this will work.  However, the Spider 2 head needs 2 x 8 Ohm inputs run from 2 x 8 Ohm cabinets( or 1 cabinet split into two(2) 2x12 groups), so there is no way to make this cabinet run all 4 speakers at 8 Ohms.

I called Line 6 support, then I verified this with 2 repair shops, and there is just not an easy way to make this happen with this cabinet.  I even considered buying another 8 or 4 Ohm cabinet, and then switch out 2 of the speakers with this cabinet and the new one, and there is just no way to make the Ohms line up and make use of the cabinet to it's full potential.  With the "swap idea", I came up with a 12 Ohm load by switching speakers and re-wiring, but then you will not get the full output as the Ohms will be too high, and you would get a net result of about 70% total volume due to the differnt in Ohms.  So again, why bother?

If you want a full stack, consider buying 2 new Line 6 4x12 cabinets that support 8 Ohms mono output in 1 jack, and you will have your full stack.  Another idea would be to get ahold of 4 x 8 Ohm speakers and swap them, but again, why bother, just get another cabinet (unless you can get a great deal on 4 speakers, like less than $100 for all).

So, there is how you make your Spider 2 head sit on top of a "Full Stack".  If you're like me, you got a good deal on the dated Spider 2 stuff, and buying 2 new cabs will cost more than what you have paid for your used Spider 2 setup, which defeats the purpose.  If you do go for it, make sure to get switchable cabinets so you have options in the future, should you decide to upgrade your head to a Spider Valvle or IV.

David