Good question, have not heard any other Harbeth speaker
but maybe the Graham sits between the two!
I felt that the Graham had deeper bass than the 30.1
But I heard the 30.1 in an unfamiliar room, and not a good room at that.
I listen mostly near field to nullify the room as best, but can not be 100% sure if the bass would be better in my living room.
The reason I like the Graham better, is not the bass, but the difference in the midrange region.
I dont think you can go wrong with either speaker, if you like their sound signature they will provide you with many staggering listening experiences. Enjoy your music, whichever one you chose.