Hey Ian, the problem is not with the BB but with the headset. It echo's because the mic on the headset is very sensitive and it's actually picking up the callers voice on the earpiece, this is why it gets worse at higher volumes. The headsets with booms don't usually experience this because of the distance between the mic and the earpiece.
I don't care for the booms either, and I like the headset to be as small as I can get it. I am currently using the Jabra JX-10 and I have not had ANY problems with echo or anything else for that matter. It is very small and the earpiece goes into the ear which prevents the echo problem, it sort of "hides" the mic and the earpiece from each other. I realize that everyone has different preferences but this has worked very well with my 8700c.
Hope this helps!