It isn't only vintage SUNN. I think it was more of a phenomenon of the late '60s-early 70s era and the common larger speakers cabs, lack of hi-tech front of house PA systems...that had many U.S. players using wide speakers "stacks" or layed down "sandwich" stacks. You needed louder stage volumes to make up for the unsophisticated PA systems/practices of the time, plus the instrument speaker cabs often were not the now common 4x12 ones that stack nicely.
Especially with the most powerful bass cabs of that era...SUNN 2000S, AMPEG SVT810, the larger Acoustic, Fender and Orange cabs....NONE really stackable on top of each other upright and still useful. Same for the early large guitar cabs of equal size, until most fell into the British 4x12 formats eventually.
Here is another perfect example....Orange Matamp 2x15 cabs from 1968 that were John McVie's from the Peter Green Fleetwood Mac era thru the early 70s. You can see in live YouTube clips that he stacked a pair of these side by side early one. Then McVie added two more, as they began playing larger venues and he changed them to layed down "sandwich" stacks.
I bought these from the second owner recently, who'd bought them in a "Fleetwood Mac Garage Sale" in the mid-80s out near Los Angeles at the band's storage facility. He said there were indeed FOUR of these for sale but he could only afford two. They actually came with the flight cases with the Fleetwood Mac logo but he tossed them for lack of space over the years (these cabs are big like a 2000S cab but actually more like a Fender Dual Showman...only 12-inches deep.) They sound fantastic and very unique. I had a couple friends out in Hollywood go check them out for me (I'm in NY)...both guys are pro bassists in bands you'd know. When they called me back after demo-ing the cabs, they said that they sounded so unique that they were thinking of having them sampled to use for recording sessions.
BTW....Some SUNN trivia from this deal. The guy I got them from also still had a Fleetwood Mac roadcase to some sort of 2x12 size combo amp or cabinet....AND A SUNN 2x12 cab stenciled with "PARIS," which was Fleetwood Mac guitarist from the early-mid 70s Bob Welsh's side band. So, I guess we add Fleetwood Mac to the SUNN users list.
Here is a composite pic of the cabs and the various components for any techies. These were custom made for McVie, so only four were ever made in this config.
McVie and Entwistle were two of my biggest bass influences, so finding these was a major score. I owned my SUNN 2000S rig before I ever realized that Entwistle was a SUNN player, so I thought it was great that my ears had picked out the sound before my eyes did. I later sold Entwistle my white '63 Fender Bass VI in the mid-80s that he kept until he passed away (and Sothebys auctioned off.).