Yep its a Jack
The 2x10 is way narrower and shallower than the 1x15, but slightly taller. I tried the 2x10 on top this evening after reading the posts, but the sound went a bit naughty. Vol, and bottom slightly disappeared.
But then one cab, the 15, is 8 ohm and the Jack is 4 ohm, but i've already been told there wouldnt be much audible vol difference.
But, as i keep making changes this evening, i'm starting to get a better sound, and though it will never be ideal due to the cab setup, its certainly improved, and i'm getting to the point where its more acceptable, ( to me ) bar finding another Jack, which is what i really want. One will turn up at some point i expect