[Matt B]

I recently purchased a set of the Sennheiser 630vb with the rotary bass dial. I really like them but sometimes I feel like the amount of bass they produce varies...without me touching the dial. They always sound good but occasionally after listening for a little while the sound seems to take on a fuller more powerful character. Even when I am listening to the same song at different times I feel that on some occasions the bass is more intense than others. It is likely psychoacoustics/perception, and if this were a regular set of headphones without any sort of circuit that adjusts bass I would know that for sure but in this case I am worried there might be some sort of malfunction...and due to this I am considering returning them. Any thoughts???

 

[Bern2]

I have these also....I like them for live recordings.  Seems like I'm sitting in the front row sometimes.  I usually have them as part of a "portable" set-up...using a Sony NWZ-A17 to drive them.  
 
I have not experienced what you described. (then again I wasn't looking for it and usually don't replay a song back again).  
The only thing I can think of is to make sure you don't have some type of auto-volume turned on from your source.  Sony calls it Dynamic Normalizer.  Noticed a "breathing" action when that is on...especially during fade-outs.  
 
Bern