My experience of the G12-65 is that it is a lovely unit, but it's too soft & smooth. I reckon a V30 or G12H would be a good match.
The V30 & G12H are both a lot louder than the 65 so you may want to try mismatched resistance drivers to try and even the levels out. e.g. a 16 ohm G12H with an 8 ohm 65.
I need to try this myself, and have the drivers here I just need to get a little motivation
I have posted this info a couple other times, but it may be helpful here.
I have a 2 x12 cab with a V30 & G12H30. It sounds very, very good. I use it with the OTS. I also have a custom built combo amp with (2) Rola Celestion G12-65s in it. I have A/B'd these two sets of speakers using the OTS. The V30 / G12H combo is far more articulate than the G12-65's. More solid lows and a crisper top-end.
I had the amp with G12-65's for about three years and I thought those G12-65's sounded great. Until....I compared them to the V30/G12H combo. When I play with all 4 speakers the G12-65's disappear in the mix. Maybe the 25W Greenback would be the best bet.
+1...I have also paired up the V30 & G12H30....installed in a repro 2x12 Fender piggyback Bandmaster style cabinet and it sounds fantastic. They are both 8 ohm speakers wired for 4 ohms and there is no absence of volume....
I have tried the OTS through an EVM and liked the tone but theres something about the Celestions that really makes this amp shine IMHO.