Are you using contour buttons and boosting the eq lows? maybe the combination of enhancements/eq/actives is doing it. but IME "some college kid" should be a little bit of a red flag, as well as that awesome price, that there has been some abuse in this rig's past, and i'd give each peice a good once-over visually AND with a known-good peice to see where the problem is. it may (fingers crossed for you) be a simple fix of tightening a couple of mounting screws on that 15, or even better, a simple eq change.
either way the head and one of those cabs should really do you ok for most stuff.
wait i just thought of something. i had the backline amp that was the lower-powered one. it only had a single 8 ohm output. could your problem be impedence mismatch?