As the starter of this thread, I feel I should update it somewhat.
I am actually now using the Sennheiser HD650 which I described in my original post as "competent but lacking soul". I have to say I have now changed my mind on this. I've now been using these for probably about a year or so roughly, and find these to be superb headphones. Whats changed? Well... the key to the HD650 is twofold. Source, and synergy. The ANT Amber is the ideal headphone amp to drive these headphones, as it offers plenty of speed and power, with good detail and clarity to boot. However, this is not enough on its own to bring these to life. These headphones DEMAND a good quality source component to bring them to life. I now have that in the form of the Musical Fidelity Digilog... which itself offers speed and power.
The Sennheisers really need electronics that can deliver a fast, powerful sound, which synergises well with its warm, bassy sound. When you can give it that, these headphones reward with a superbly natural sound, not boring at all, plenty of bass, plenty of power, and plenty of detail. They sound as close to a pair of loudspeakers as I've ever heard in headphones, and I have to say, that no longer do I regard headphone listening as a compromise, I will sometimes listen to these in their own right in preference to my speakers, just for a change.
I stand by what I said about the Beyerdynamic DT531... they are a superb headphone for those on a budget, and ideally suited for casual headphone listeners after a fun sound. Comfortable and lively. Why pay £200 for a pair of Grado's when you can find DT531's which sound just as good and are far more comfortable for £50 second hand?
I have also revisited to Beyerdynamic DT880. I still find them to be somewhat bright and lacking bass. This is not an enjoyable sound for me. However, I did hear them on a cheap chinese Valve amplifier, and I have to say these are ideally suited for valve amplifiers. When paired with the ANT Amber, the result was a harsh sound.
I also tried the Goldring DR150. I found these to sound a bit "hard" and unpleasant to listen to. Again, I think this could be a synergy thing. However, even despite that, I cant see these being as good as the Beyerdynamic DT531 in any respect.
Finally, I revisited the Beyerdynamic DT990. These have a fairly bassy sound, however, I think at the expense of the midrange which sounds a bit distant, and the treble is rather bright on these. Overall I didn't feel it to be a natural sound, and I have to say, I felt the DT531 were once again the better compromise, a little more comfortable, and just more fun and more balanced overall.
What prompted me to move away from the DT531 that I owned and loved for so long? Well the problem was I bought a new source component, and unfortunately it did render the DT531's a little on the strident side, making them a little tiring to listen to. I tried all of the previous headphones in this post before giving the HD650 a third chance!
For a long time I regarded the HD650 as a good but dull headphone, but I was so wrong. In the right system, they are simply great headphones, and anyone who thinks they are boring has not heard them in the right system. I should know, because that was me in the past....