Hello, Can anyone tell me the difference between the three amps in the Hartke Kickback combo series? There's a 10" speaker model, a 12" speaker model and a 15" speaker model. Since all 3 amps are rated at 120 watts, I'm "assuming" they all put out the same volume? So, why should I pay $150 more for the 15" model over the 10" model? What type of sound do you get from each model? My local store only has the 12" model so I don't have a basis of comparison. Thanks!