I was actually able to do a similar combo you are thinking about a couple weeks ago.
I was comparing a 12/6 and 12sub to my SVT212AV. The 12/6 did not have the 1" driver as the guy who built it just didnt like it the sound.
The amp loved the 12/6 by itself. The low end extension as you know is pretty crazy for such a small cab. In my situation, competing with 20-50 watt guitar amps with single 12s, the 12/6 was plenty. With the 12sub as an extension wouldve been too much low end honestly, and just an extra thing to schlep. If you needed to carry a room with no FOH, i could see bringing the extra cab, but that isnt what the PF50 is for.
The amp drove the cabinet perfectly well from what I could tell. I didnt run a big amp through it to see what it would really do. It was plenty loud for my needs. Our band just isnt terribly loud.
As far as I can see, the PF50T into the 12/6 would be about as good as you can get in the tone and stage volume department in a small package. If you actually need 600 watts, I dont think itll be the amp for you in a live situation. I dont care how efficient those speakers are, youre in a wattage problem. A V4B would probably do the trick, or an SVT obviously.
Cliff notes, you cant under power a cab. If it sounds good, it is good, and the PF50 sounds great through the 12/6.