Just a quick update, I went with an ENGL slanted standard 4x12 with V60s and I love it - it just fits the amp absolutely perfectly, no v30 cab can come even remotely close and I tried ENGL's pros and my own fat and smooth Bogner 2x12 but with the v60s it just feels, sounds and plays RIGHT. With v30s I had to turn gain up to 2 or 3 o'clock to get a sort-of nice and fat tone and it was still mostly fizzy, totally ridiculous. As if amp and cab are playing different frequencies or are mismatched somehow. With v60s I turned it all the way down between 10-12 o'clock and get a nice, fat, rich tone with those gorgeous lower-mids. So, yea, definitely no v30s for the Blackie in my opinion. (I might try the greenbacks too some time out of curiosity but I was worried about the power-rating of the cab probably not being enough for peaks from the Blackie.)
I`m glad you found what works for you
I have a hard time finding whether those speakers really are "celestion v12 60s" and where the heck I can buy them to eventually swap them into a PRO 2x12. Or I tip-over a slant 2x12 standard, they do have that.
I believe V60s are made only for ENGL and they are not available for sale.
So the only way for you is to tip-over 2x12 standard cab (as you have written) or to wait till someone would upgrade the standard cab and would like to get rid of them.
They pop up from time to time on ebay.