I never did compare them both intentionally, but I did listen to them on and off at the same time. I'll say RE400B benefits more from HM700 than RE600B. While RE600B is still ahead, RE400B is very close. If you are thinking about getting the max bang for the buck, I'll say you should go for the RE400B package as I don't think you will lose much over the RE600B package. Between the two, RE600B is slightly less warm / more neutral and better on detail retrieval. RE400B on the other hand has better texture.