I have two marshall 4/12 cabs. One is a 79 or so slant with british reissue greenbacks (at least I think they are reissue). The other is a 71-2 straight that I loaded with WGS reapers and old celestion g12m blackbacks in an x pattern.
The 79 slant is significantly louder than the 71 straight and also seems to have a fuller bass response, which defies all logic since the straight has the reapers which should be bassier than the greenbacks. I just pulled the back off the 71 and checked the wiring and its good and I used the black and white cloth stuff you get from mojo.
Any ideas?
Here's another thought to ponder. I also have an et65 and a vet 30 hanging around that I could use if need be.
If that cab is a '79, I believe that it should have the real-deal greenbacks ... not the re-issues. If they have been treated well, those are some awesome speakers. I'm not sure what the "black backs" are that you referenced ... just standard modern G12's? If so, then yes, try yanking them and replacing with the ET65/Vet30 ...
BLACKBACKS are just greenbacks with a black sticker. I'm not totally sure but I believe they were produced in the 70's. These are g12m's
I will pull em and stick the other wgs's in and see what happens. I didn't know if there was something else I could be missing that would produce less output.
Also check the whole series-parallel vs parallel-series thing ... I beat that horse to death quite a while back on my blog ... just ensure they are the same ...
And, check back in here with your results ... I'm very curious!