hello, I haven't been playing bass that long. I need advice on which amp head to buy. I have 2 cabinets. One is the hartke xl series 4.5 4x10 cabinet, which is newer, rated at 400w at 8ohm. The other is an older hartke transporter 1x15 also rated at 8ohm. Can some one suggest a decent amp to power those cabinets? I was thinking about the ampeg pf-500. I tried it but it doesn't seem to give those cabinets very much volume. Is there a better amp to power those cabinets? I have a musicman stingray active 4 string bass. if that helps.