Pros: Durable, well defined, crisp, precise, isolating
Cons: brittle, bass shy
I've owned these headphones since 2002, yep, they have lasted me 10 years. Used all this time in my home studio I thought they were rather expensive at the time. I see they are now under $100 what a bargain. Most of the black vinyl that covers the outside of the ear pieces have completely disintegrated but the foam is still fine I've had no problems with the cord, plugs or head bracket. Its amazed me how durable these have been. They are great for their intended use as isolating recording headphones and as long as volume is kept at a reasonable level won't bleed too much into microphones. Over the years DACS have come and gone, I currently have an Apogee Duet and run that via a firewire to thunderbolt adaptor. They are comfortable to wear for hours and cause little ear fatigue if kept at medium volume. If you pump these phones you will end up with ringing ears but that will happen with most ear phones.
With 1/4" adaptor off and plugged into an iphone they sound great compared to the apple buds.
Do they lack in bottom end? If you crank them up the bass sounds pretty good, of course, you'll also be cranking up the upper mids which these phones are famous for. So the boost around 1-4K assuming you have the original 7506 and not a fake, mask the the rest of the spectrum. For loud listening, I'd advise a gentle reduction from about 3k on or a slight smiley face from 1K-4K. But if you're going to listen loud you'll get ear fatigue no matter what you do, whatever phones you use. I find for recording they represent the bottom end nicely, sure they lack weight down below 55HZ but below 80HZ and above 14KHZ human ears are less sensitive. Thats why we love to boost those frequencies. :-) If you record or listen to rap/hip hop/DnB or even bottom heavy rock, you may need some higher end phones.
I use a lot of synthetic sounds and low sounds when doing rap and find the Sony's good as long as you don't get heavy handed below 100HZ otherwise the mix won't translate. I also have to not be shy with the upper mid range otherwise it will lack energy. I always have a spectrum analyser on the mix bus so I know at least visually whats going on. You can't always trust your ears. That and my near fields, that go down to 55HZ are enough for me. Oh and the wall that goes down to 42HZ helps. :-)
If I was buying a set of listening head phones I'd go to a store and test drive a few famous pairs I've read about on head-fi and then decide but for tracking, the Sony MDR 7506's do the job.