I have been using a Ampeg PF 500 head and a Fender rumble 4/10 cab. I replaced the speakers with Legends, giving me 600 watts at 8ohms. I recently played a gig and had the Amp vol cranked to about 3-4 o clock and felt like I had no power coming out and was hard pressed to get a good clean sound. All other knobs set at pretty much flat. Any thoughts? Do i need a new rig?