I converted a 1965 4x12 into a diagonal 2x12
If you mean the 1965A or B from Marshall, those were 24" wide 4x10" cabs.
Had the A and B version of those, while I also had the 1966A (2x12" diagonal) and a 1966B (which actually was a converted 1965B with a new Siberian birch baffle).
The 4x10"s (G10L-35 speakers) had a nice tone, quite smooth and punchy, so they helped shaving some of the bitey nature of my 5150III 50W and I gigged with them too. But in the end, I preferred the bigger push of the 12" speakers, so I sold those both and kept the 1966s.
I mostly gig with a Mesa Vertical 2x12 (G12-65 in the top, Mesa V30+beamblocker in bottom), which gives me the right balance of tone and portability. But I hate the look of the 'T' as well, so I don't use it with a full size head.
The other guitarplayer in my band does, and when he's not schlepping his Marshall 4x12 cab, he uses a JVM410H with an Engl 2x12 vertical...Creates the T. I stick to the smaller width of the EVH 50W.
Only for the bigger shows, I'll take out my Engl 4x12 Pro cab, which is more than 1.5 times heavier to schlep around than any Marshall 1960xx cab.
Sure it looks cool, sounds big (I replaced 2 V30's with vintage G12K85's for even more low-end punch), but it's a 120+lbs cab for cryin' out loud. The Mesa 2x12 vertical is 75lbs (close to a Marshall 1960A).
And as some people mentioned, once you start close micing it, the difference becomes naught. Closed back cabs that is.
Oh, and adding proper absorption panels inside a 2x12 make ALL the difference in getting it sounding bigger.
Besides, I just like the look of 24" heads like Mesa Marks, Engl Savage 60 etc., which fit perfectly on those diagonal 2x12s....