Hey Greg . Doh, I missed the 4 ohm reference! I wouldn't worry too much about going past 12 o'clock on the gain and master as long as speakers aren't complaining. Unless you are hitting the strings really hard the gain control limit led should remain off at 12 o'clock with a typical output passive bass. Although the cab is rated as 800w this is a thermal rating only and the LM800 extra power may just end up being dissipated as heat rather than generating more volume. The 500 watts from the lm3 may already be as much as the cab can convert to sound energy. If you are maxed out on the amp and the cab isn't loud enough the only solution is probably more speakers ie use two 8 ohm std104hf.