02 yukon sub

What size is the factory sub and is there a decent replacement using the original location?

I have an 02 HOE and it is 8". Crutchfield has a few direct replacements but you are still limited by the poor factory amp and HU. Most suggest replacing the HU, add a sub amp, and new sub. I just replaced my HU and 4 door speakers with huge improvement over factory sound. I am planning to bypass the factory amp to see if the sound improves further or not and then decide on what to do for my sub.