Ok try to follow me on this one. I have and extra speaker combo cab which used to house a 15' speaker. I disconected the amp and want to replace the 15 and add it to my stack. Let me first explain that I know almost nothing about ohms. I have a Hartke 4000 head. Which is 400watts at 4 ohms. For speakers I have an Ampeg cab with 4 10's. I'm not sure which model but I know its rated at 800 watts at 8 ohms. Now my questions are this. What kind of speaker will I need for the 15'? What am do I need as far as ohms and what not are concerned for the 15? Thanks for your help.