Been meaning to switch out one of the g12-75's for a v30 in my Marshall 2x12 for some time so I finally did it last night. Turned out to be a bit of a soldering mare, not the lovely quick connect stuff on the newer ones. I know it's been done time and time again on here, but hey, the more clips the better 
So just got back from work and thought i'd chuck a mic in front of it to see how it turned out, and it seems to be pretty nice.
Here are some stock settings on the 5150 going into each speaker, sorry about the crap riff. We're talking 5150 on post gain 1 here too, so no ball crunching tone today.
This is what left and right speakers sounded like before I did the job. I elected to take out the right speaker, as it appears to sound like shit (which i'm sure you'll agree on!).
Left Speaker G12
Old Right Speaker G12
And now here is how it sounds since the change (with the same left speaker):
Left Speaker G12
Right Speaker V30
I'm actually wondering whether it's worth replacing the left G12 with a fresh one now to complement it... ?
I like the smoothness of the old marshall speaker so it will get used for rock stuff. V30 for the more metally stuff I think.

So just got back from work and thought i'd chuck a mic in front of it to see how it turned out, and it seems to be pretty nice.
Here are some stock settings on the 5150 going into each speaker, sorry about the crap riff. We're talking 5150 on post gain 1 here too, so no ball crunching tone today.

This is what left and right speakers sounded like before I did the job. I elected to take out the right speaker, as it appears to sound like shit (which i'm sure you'll agree on!).
Left Speaker G12
Old Right Speaker G12
And now here is how it sounds since the change (with the same left speaker):
Left Speaker G12
Right Speaker V30
I'm actually wondering whether it's worth replacing the left G12 with a fresh one now to complement it... ?
I like the smoothness of the old marshall speaker so it will get used for rock stuff. V30 for the more metally stuff I think.