I'm not personally real up on Sunn transistor amps, however there have been some pretty compelling arguments made against the stability of the Concert output stage running a 2 ohm load over the years; by IMO some very tech savvy forum members.
I know this though; in the real world of professional sound reinforcement, running serious high dollar/low impedance stable amps, intentionally planned 2 ohm loads are avoided like herpes. These are amps that will often run stable bridged with a 2 ohm load, far more "bulletproof" than the amps being discussed here.
There are myriad reasons that running a non OT transistor amp with a 2 ohm nominal load is not a good idea, but to summarize it's real close to a dead short. It leaves little margin for error. Taking into consideration the nature of speaker impedance ratings, the amp may regularly see loads that are a fraction of an ohm; too close for comfort in the pro world.
A happy amp is a good sounding reliable amp, and a 2 ohm load doesn't make for a "happy" amp.
From some posts I read, it almost seems like running 2 ohms is like a "bragging" thing, when in reality the increase in actual dB between 150 and 200 watts is insignificant. For all intent and purposes virtually inaudible to the ear, requiring accurate measuring equipment to notice; maybe <1 dB, and IMO the potential risk FAR outweighs any potential gain.
While it may be convenient to daisy chain a pile of cabs together in parallel, you can generally run as many speakers as you want with judicious use of series and parallel wiring, so the excuse that 2 ohms is required to run multiple cabinets doesn't really hold water either.
Sorry for the little "derail" interloper! 
Ohms are still somewhat new to me, so forgive me if I sound clueless...so I gather from this info it would be better to run the head at, say, 4ohms? I will most likely be using it with an Orange PPC412 cab, in which case will I have to adjust the impedance level on the cab to suit?
Transistors will output more power (and more heat) the lower the load impedance goes. Regretfully they also become less stable the lower the impedance goes to the point where they will go Chernobyl and just fry your amp.
There is usually a minimum recommended/specified load impedance for any individual amplifier by the manufacturer; usually dependent on the robustness, topography, output devices used etc. of the specific amp.
The debate here is what that minimum is for a Concert series Sunn, more specifically whether or not the Concert series is "stable" running a 2 ohm load. I would say that the consensus among the tech types on this forum is that 4 ohms is a much better/safer idea with the Concert.
What my derail was is pointing out that "running 2 ohms" somehow has become a hip catch phrase in certain circles, and makes an
insignificant difference over 4 ohms in the SPL of a rig in the big picture anyway.
So what's the point of "running 2 ohms"?
*
IMO if you want LOUD adding more speakers rather than stressing your vintage amplifier to the point of catastrophic failure.
I'm not aware of a way to
adjust the impedance of an Orange PPC412, as far as I know they only come as a 16 ohm mono cab or a 8/16 ohm stereo cab and you would need to rewire the cab to change that.
It's probably 16 ohm drivers in series parallel so you could go all parallel to get 4 ohms.
*I have run 2 ohms with my SR gear, but only in an emergency such as power amp(s) going out at a gig.
I run enough amps to keep
everything at 8 ohms as much as possible (not always possible) in order to keep everything fat and happy and maintain ample room if I have to "double up" in case of failure.
