I think the main differences can be attributed to their names:
- Harbeth - named after the brother/sister (Harry and Beth) who first designed and built their bookshelf speakers in their uncle's lonely Wessex manor house.
- Graham - named after founder Paul's love of gravy and ham (GRAvy and HAM) breakfasts in his Devon area farmhouse as a kid.
- Spendor - named after the husband and wife founders (Spencer and Dorothy) who started operations up on the north midlands. Also the birthplace of Def Leppard.
One of these is true, btw.