I recently bought a used Carvin Pro Bass 150 head. I tried it out with my friend's 15" Crate bass cab, and it sounded great. It seems loud enough for what I do (smaller gigs at parties and clubs and stuff like that) and kept up just fine (with the volume at about 2) at practice with my loud drummer and my guitarist with a big half stack. It's rated 100 watts at 8 ohms and 150 watts at 4 ohms, so I'm now trying decide what cab to run it through. I would ideally like to run the head at 4 ohms to get the most power out of it, so I was thinking maybe a 2x10 (8 ohms) and a 15 (8 ohms) run in series to cut the impedance down to 4 ohms. I would rather have a 4x10, but most of them are 8 ohm cabs, and also I dont even know if the head could easily push a big 4x10. I have heard that there is such a thing as UNDER-powering your speakers too. The head has a line out and 2 speaker outputs, and we have a PA we use, so I figure that will help, but I still cant decide on which cabinet I should get, so any advice is appreciated. Thanks.