i played 2 hours with my Vetta2 HD's impedance at 8 Ohm on a 16 Ohm mono 4x12" cab (not mine!): can i have made serious damages on my head? the head impdance was set lower than the cabinet impedance, i just forgot it because i wasn't using my gear...
as the vetta is a solid state amp, usually doubling the speaker impedance would be ok... it would just lower the volume.
if it sounds all ok, and is still loud with the correct settigns, you should be fine