Just thought id add my experiences.
I have a mid 4000 serial pair (I considered to be Bass Heavy). I started hearing distortion in the right channel so sent them to RWSoundSystems in Germany to have 2 brand new drivers added (They arrived a few weeks ago) . I was initially worried about the light bass and purchased a sub woofer to improve this area. This helped me see exactly what i was missing, the new drivers arnt as "warm sounding" as the old ones. They sound more precise and realistic to me. The only downside I'm experiencing so far is is a lack of impact on the bass. I think this will get better as the headphones burn in. In the end i realised i wasn't missing as much as i thought i was and took the sub woofer back. I used to have a very warm system of a Melos Amp, Copper Cables and Grado PS-1s. Initially i loved it and i really enjoyed listening but i decided the bass just didnt sound realistic and was colouring the music too much.
So i switched to a more neutral K1000 rig and it was exactly what i wanted. The newer drivers have taken a little more warmth out of my system and to me, its another improvement.
I'm happy with what I've got now. For the price i paid for the new drivers i could have almost bought another pair of Bass Heavy K1000s but providing i get a little more impact in the future, I'm happy i went for the newer model.
There are my opinions and my SAA cable and Amp might be complimentary to these new drivers which could be responsible for the good results. They really do sound great. I'll try and post pictures soon because they look slightly different to my older drivers. ie there are usually little wires on the inside and the outside of the drivers (visible after you've removed the grills). On my new pair there are only wires on the inside?