[Kodhifi]

I got into a discussion with someone about 701's vs 880 and was talking about how much forward the 701's bass is in comparison. Long story short they disagreed so I went and pulled the frequency graphs and sure enough my observations were true, but only comparing 250ohm 880 (what I own) to Q701. The original K701 had a more anemic bass response which would have put it on equal footing with the 880 but then I added the 600 ohm 880 into the equation and I was shocked to see it's frequency response was very different from the 250's. It had about 4db more bass along the same curve from 150 down to 20hz.
 
I know a fair bit about electrical engineering and audio engineering so I already knew that the higher the impedance on a driver the more accurately it tends to reproduce sound, with the low impedance drivers tending to be a little more sloppy in their response curves but that doesn't change a whole lot from 250 to 600, not as much as from 32 to 250 anyway. But I never thought about the sound signature or if I did I assumed it was minimal.
 
 
 
 
So how do the 600's stack up to the 250's? I really like the 250's but my only slight dislike is the underemphasised bass which always leaves me wishing for a little more. Especially on gypsy jazz and jazz standards where the double bass just sounds put off in the corner.
 
Worth it to buy a 2nd pair of 880's for better bass?

 

[NA Blur]

Sonically they sound the same to my ears.  Depending upon what gear is driving the headphone the impedance may become more important.  In most cases I error on the side of lower impedance and higher sensitivity this is because as the impedance of a headphone rises the current draw from the amp varies with frequency.  You may get better control of the driver with a higher impedance headphone, but you also going to need more voltage to get the loudness you want thus turning up the volume knob.  Systems will have their limits as to how much voltage you can extract from them such as portable devices so at some point a higher impedance headphone becomes problematic.
 
Here are some charts showing the opposite effect;
http://graphs.headphone.com/graphCompare.php?graphType=0&graphID[]=2131&graphID[]=2231&graphID[]=713
 
See how the DT770 ( 600 Ohm ) has less bass?  My guess is that the load is causing issues at the fundamental bass frequency making it harder on the amp to drive the headphones.
 
Here is the DT990 Case:
http://www.headphone.com/headphones/beyerdynamic-dt-880---600-ohm.php
 
They are almost identical so any differences you hear are most likely caused by driver control, current / voltage limits, and driver differences from each type of headphone.