Have the Soloist SL as well! :)
Surprisingly, I have NEVER used the Signature DJs with it! LOL
Might as well try it sometime...
I will probably set mine up for sale sometimes next weekend. While it wasn't quite the right headphone for me, it has been a good and learnful experience. :) Although I have to say my fears I had also turned out true, further strengthening my vision that you can if you are exceptionally dwelved into knowing your personal preferences and knowledable in the hobby (know how this and that affects the sound we experience) and back this up with multiple user reviews as well as scientific measurement to form a picture of how the headphone may sound like, it's a pretty solid ground to base on.
I had this feeling I already knew how it would sound like and it was pretty much what I expected, I had hoped that it wasn't quite true I guess regarding the mids/highs balance and how this effect my listening experiences and soundstaging etc. I felt that I pushed up the volume a little higher with these headphones to get the mids up to the level which I wanted them to be, the foam mod helped a lot in this regard as it tamed down the highs so it wouldn't result in painful highs at the point the mids were in-my-face too but I felt I still didn't win anything over Q40, possibly tiny bit improvement in terms of soundstage but it's not enough to warrant the discomfort issues I had though.
I keep wondering if there ever will be any true "upgrades" for Q40 for me, gonna see how the bassy new Ultrasone 840 or whatever it was will be like as it's said to be very bass driven without emphasized highs lacking the clarity of Sig DJs (which I read as less emphasized highs which I take as a good thing for my taste).