+1, the 1212/6 is a 4 Ohm cab and will sound fantastic with the walkabout. A 15/6 is only a little smaller, but has both less cone area and more impedence, so it won't go as loud as the 1212/6. The downside is that the 1212/6 will cost more than the 15/6, but I think it will suit the amp better.
Unless of course you add a 15sub...

Then again, if you are going the 15/6 and 15sub route it may be better to get / build a 1515/66!

And then add a 15sub again... or....
maybe just a 1212/6 after all.
