Alright! It's tax return season so I have purchased my first head/cab and I have decided to grab the Ampeg PF-500 head. http://www.guitarcenter.com/Ampeg-P...18-i1809880.gc?source=4WWRWXGP&kpid=106220718 it pushes 300 watts at 8 ohms and 500 watts at 4 ohms. Alright so I also picked up a Hartke 4x10 cabinet (which sounds great!) which is rated for 400 watts and is 8 ohms. I'm wanting to get another cabinet (I'd really like a 15 inch speaker.) So here is the real question. I have the amp running one line to my 4x10 cabinet which is rated at 8 ohms. It has a "Parallel" output jack beside the input. What would the 15 inch cabinet need to be rated at in terms of watts and ohms in order to make my impedance 4 ohms?