I'd like to thank everyone who contributed to this thread.
I recently purchased a pair of HD681 EVOs, listened to them for about 10 minutes and then set to modifying them based on observations and issues.
So far I haven't put in the filter to drop the bass (I'm a bit of a basshead) but it might become annoying enough to warrant cracking them open again.
I have a pair of Philips Fidelio X1's and Beyer DT770 250ohms for comparison, all running from an Aune T1.
Some day soon I'll replace the Aune with a higher quality amp but it's doing it's job well.
Someone commented that the EVOs benefit from decent amping and I'm inclined to agree. Separation is better as is the sound stage and the overall extra energy is nice. At work I've used them straight out of a DAC and they're totally agreeable from that as a source.
I ripped out the male dongle 3.5mm plug and replaced it with a quality female jack, replaced all the wiring and removed the felt from in front of the drivers.
I also glued a strip of "pleather" to the top of the head strap to keep it from ripping. This was both cosmetic and functional. The head band now looks quite a bit more money.
The
jack I used looks absolutely stock and holds 3.5mm plugs strongly.
Removing the felt definitely helped. I like treble so removing that veil was good to my ears. There are of course tracks that have more harsh treble in them and this just accentuates that more so I try to stay away from them. So far so good.
Interestingly there are certain things that I think the EVOs do better than my X1's. Something about how they present the sound stage coming from the Aune has made me interested to swap tubes again. I've been using a Voskhod 6922 equivalent for a long while and I'm curious to hear how my Amperex and RCA tubes sound with these cans. I feel like a lot of the audio is more centered and colored slightly differently than what I'm used to and I can't yet tell if it's the tube or the EVOs.
The highs are obviously more "sparkly" and peaky and in ways, outshine the highs on the X1's. Things are more crispy and I'm noticing small differences I hadn't before. Possibly the elusive "microdetails". The highs on the X1's are known to be rolled off and the EVO definitely extends the highs. Of course the bass is way more than what the X1's put out which is already more than most "audiophiles" care to have. As far as the quality of the bass, I still haven't come to a conclusion yet. There's resolution and texture to it but it's definitely still more "boomy" and softened than more expensive cans. I can't complain about the extension though, the sub bass is pretty amazing.
I'm not a fan of disassembling these things. Not because it's hard, it's because reassembling them is not cool. The elastic straps for the headband are scary to push back into place because you have to push them in from the back with something narrow and not pointy. Even using the tip of needle nose pliers I managed to puncture the rubbery elastic on one of them. This is where the Superlux cost cutting comes into play. I'm still amazed they used the tension rods inside the plastic tubes as conductors.
I think the wiring attached to the male 3.5mm jack is extremely small, probably 30 gauge wire. My assumption is that if you have trouble with one of the drivers, it's due to a bad solder joint or a broken wire. I replaced all the wiring with 26 gauge just for good measure.
I'm too lazy to post pics right now. Also, they're on my head.