Legend has it was Pete Townshend who requested the first Marshall half stack. Marshall at the time only made an 8x12 cab for big cabinets, and Pete wanted it made into two 4x12s to make it a bit easier to deal with.
Yup, right after he gave up Marshalls. Another part of the legend is that one of the reasons he started using Hiwatt was that Jim Marshall wouldn't give him any more amps on credit, as he had a habit of destroying them, getting them on credit from Marshall (a small company at the time) and never paying for them.