Don't worry about where the volume knob is set until it won't turn any more or it starts sounding too ugly.Ok team, I returned the HD500, mainly due to it not having an extension cab option. Also, it wasn't quite loud enough for my needs.
I exchanged for an Ampeg RB210. It "seems" a bit louder than the HD500. But, I'm running it at almost 75% up the volume knob. If this one doesn't work out, I'm just going to bite the bullet and get a head/cab option (which I probably should have started with from the beginning).
Looking at either the Orange Terror Bass or Little Bass thing with the 4 ohm SVT 212AV cab. "Should" be more power in that setup.
More speaker cone area if you need it - Ampeg PF-115HE or PF-210HE run about $500.