Others have noted that the bass adjustment is subtle. And it is subtle at first. In my case, what I was used to was loudness
controls; something like the bass boost on my car radio. Those give a big first impression -- but after a few seconds
it's apparent these controls apply changes to much more than just the bass. The low mids are shifted and almost everything ends up
sounding muddy. IMO.
But the bass adjust on the Rx MK-3b seems really just about the bass. Actually it is wonderful that it does not hit me over the
head. I listen a good bit with my AKG K-702s. I always use the bass control with these -- it nudges those low bass notes
just as needed, but without detracting from the beautiful clarity of this model. Most of my listening is with this combo:
K-702s and Rx MK-3b with just a bit of turn to the bass knob. Wonderful feature.
No surprise, but I never need to use the bass adjustment with my Beyerdynamic DT990/600s. No reason to add bass there -- they've
got plenty. And the Beyers sound very nice with this amp... so I am also using these headphones much more than I did previously.
All listening is single-ended at this point. Hope to get a balanced cable for my HD-650s this summer... and looking forward to it.