Wow just looked that thing up - that's a monster head!
When I got my Orange several years back, I had found some helpful info on their forum. I couldn't locate the original post, but I found it quoted and being discussed on some other forum:
http://www.bossarea.com/forum/topic.asp?TOPIC_ID=3960There's some good info in there, but to paraphrase it:
Solid State amps: Safe with rated load or with any higher impedance speaker load (up to & including an open circuit ie infinite ohms.) The amp will develop less power as the impedance increases. Do not short circuit (0 ohms) as this spells sudden death for the output transistors, but they'll sit happily all day with no speaker load applied.
Valve amps: Match impedance if possible. If mismatching, it is safer for the amp to mismatch low (e.g. connecting 2-ohm speaker load when 4-ohm is the stated minimum). This will wear the valves but the amp shouldn't suffer. A short circuit (0 ohms) is normally survivable. It's the circumstance where you try to put a signal into an open circuit (i.e. not connecting any speaker load, creating virtually infinite ohms) that is a real killer.Based on that, it doesn't sound like it would be advisable to run an 8 ohm load there.
What kind of cab do you use? Depending on the impedance of the speakers, it might be possible to rewire it to match the impedance requirements of the head. Either that, or it gives you an excuse to buy a second cab.