Yes, both of those things. I was going to say that a lot of what folks call "burn in" might really be just getting used to a different sound.
I also have two pairs - hence my comment that there wasn't much change. I think most of the 'burn in' effect is psychoacoustic, as with most headphones.
I would have normally agreed with you that "burn-in" is people just getting used to the sound, but I have two pairs of 7520 here. (
picture from when I tried the Alpha Pads)
One which has been used several hours a day for almost two years now, and one which is the same age, but unused. I tested them both when they were new, and the second pair went back into the box. I then tested them about a year ago to see if there were any differences, and was surprised by the results.
Yes, I liked the 7520 so much that I bought a second pair when it seemed like they were being discontinued - removed from sale in Europe due to stupid laws because of their high sensitivity and power handling.
While the pads may contribute, I also swapped the pads over when I tested them after about a year and there was still an obvious difference in the frequency response. A tighter low-frequency response rather than the initial somewhat bloated-sounding bass, and smoother high frequencies.
After the Alpha Pad experiment recently, I ordered a pair of new Sony pads for them, and they did not really change the sound either - not that I expected them to. There's still a difference between the well-used and unused 7520 even though they sounded the same when new.
I ordered the replacement pads, not because I needed them, but because trying the Alpha pads made me realize how much I do like the stock ones, and did not want to find that they were unavailable if I did need replacements.
And I think it's a testament to how good these are that I haven't had any problems with my current pair other than one of the joints creaking, which was easily fixed by opening them up and applying some lithium grease to the joint.
I think that's the longest I've had any pair of headphones without encountering some kind of real problem, whether it was the earpads splitting, the cord becoming brittle and one of the channels going out (I'm looking at you Grado) finding them uncomfortable to wear, too heavy for all-day use, or finding that I am unsatisfied with the sound and start looking for something better.
They use a standard 3.5mm jack, so if the cord ever did need replacing, it's easy to do.
While I like the coiled cord, I do understand why people might prefer a straight one.
It's a shame that Sony do not sell the long & short straight cords that the Z1000 came with, because I'd prefer to have the locking connector if I did replace the cord, and there are times where the short cord might have been useful - I just used another 3.5mm cable instead.
I was looking at getting the Z1000 pads for these as they are deeper and found them more comfortable than these 7520 pads but trying to find Z1000 pads is like golddust even though the so called official supplier Sony refer you to for their parts centre has them for a crazy price and does not show you any picture or the model headphone it is for just their own cat number so very pessimistic they would arrive as the correct ones.
I think you have that backwards. If I recall correctly, the 7520 pads were slightly larger than the Z1000s.
[...] right away I felt the bass was bloated. Compared to them, my 7506s sounded better to me, with a much airier sound. Then I listened to a couple tracks from Beach House's Teen Dream. I picked up some nice detail and stereo imaging, but I still felt like the bottom end was crowding the top out. That was the end of that first mini-audition.
Maybe EQ them at the beginning if you really think it's necessary (I didn't) but they honestly do change over time, and that bass really does tighten up and the treble smooths out a bit. (without losing its detail/resolution)
The overall sound does not change so much that I'd say you should keep them if you simply don't like them at all, but the two changes I mentioned are very real, and the headphones seem to respond very well to EQ if you want to try that.
I don't know what your other equipment is, but another thing which contributes to the LF response, and made quite a difference was when I upgraded from using the headphone output on my devices to getting a dedicated headphone amplifier. In my case, I bought the Benchmark DAC2 HGC.
You shouldn't need to buy anything that high-end, but a good amplifier with a low output impedance (<3 ohms) should help tighten up the low-end even further, and it's really nice to have a DAC/amplifier where there is no background noise at all, at any volume.
Here is their whitepaper on the subject, testing with Sony's MDR-V6 and Sennheiser's HD650: http://test.benchmarkmedia.com/discuss/sites/default/files/Headphone-Amplifier-Performance-Part-2.pdf
