One of my RM40 clients purchased the Ref 9s and said the same thing.
However one week of playing the amps and he has reported the "great bass" is now as good as he remembered, when he first heard it.
Whether you beleive in "burn in", break in, or whatever, this has been reported by some.
I'm burning in a P8 for a friend of mine right now, and I have noticed that while a little thin and maybe not quite as "bassy" when I first fired it up, it is now much richer tonally, and has that bass body that we all like.