That's the one.
But, I didn't order up the tube because I suspected it may not be needed at the volume levels I play at if the speaker was changed to a G10 and that appears to be the case.
Observation 3: On both channels (this is especially relevant on the red channel) the fizzy/trebbly/glassy nastiness has gone. This is what I was mainly after and it has worked. Before, I had to move the SG's volume down a notch to avoid the nastiness (aswellas dialing the amp treble down to 10am), but now the full guitar volume sounds great with no hint of the fizz - this is really what you want when rocking out; I can now go to 11 (1 full amp volume + 1 full guitar vullume = 1 and 1 which obviously equals 11). I've moved the amp treble to 11am - I now have better range/usage of all the controls.
Observation 4: The midrange focus has moved down a bit, and I like it. To my ears, while at levels where it's not breaking up (much) yet there's a new gravely grit to the sound which I like; not full on OD or even early break-up, but rather a foundation of grit.
Observation 5: There's improved soundstaging detail; notes are more distinct and separated in the musical space.
Observation 6: I will need this confirmed by the household noise complaints department, but I've turned the volume up 1 notch and feel that I get the nicer sounds that one wants by driving a Marshall a bit (at least a bit), but at lower volumes - if that makes sense... The noise shape has changed to one where I can get the sounds I like better at a lower apparent volume - I think this is a function of the midrange focus change and the removal of treble. Also, it's got to be linked to speaker efficiency, etc.
Once she goes out (it's Saturday, so is surely gonna go out shopping??? Maybe I should give her some $ and ask her to buy herself something nice; from a shopping centre over the other side of town...) I'll switch to full 5w power mode (yes, I know that sounds pathetic...) and turn the volume up then we'll really see where we are.