Why does he want the 6x4 versus say a Galaxy or Ranger?
Now, before the thread erupts into a forest fire ...
The AT-6666 is a small mobile - but then too, he wants to run power - theres' the problem.
The dynamics...
One can shoot more power into the other than it's worth.
You can fix that by removing 1 of the Finals in the AT-6666 - then you're stuck with a dog until you put that extra/Original part back in...
The "dog" references being size of power comparison...
So, he has to make choices - if he wants to run a radio both Stock and Barefoot and then kick it in - then the task becomes converting a Lo-Rider Drive amp into a 4x$ - er, 4x4 Monster AMP truck capable of Flat-sucking Deep-Cycle Hi-Lo Lift batteries into Lead and sulfites and Turning Firestiks into Kindling and ash ...
Else just get the AT-6666 pull the cover plate off and unsolder the legs of one final and lift its legs off so it doesn't make contact - then you're equal (in power - don't fret over the S/RF-meter readings) and he can run the Amp nearly all the time and skip the line for the Concessions stand...
Then there's the old Adage...