I dug out a pair of K340 from my closet, under a ton of spare cables, where they've been hiding for over 15 years. I only needed headphones with a standard stereo plug to use my old Singlepower SDS amplifier as a 6SN7 tube tester (listening tests after the MaxiPreamp tester). BUT I started noticing some of the better 6SN7 were sounding good. REALLY good. And oh my, I found myself falling into these headphones again!
My Stax 009 / DIY T2 setup has been in stasis a while, until I get around to dealing with its heat issues. That's my ultimate reference. In the meantime I have HD650 (w/ Silver Dragon), a newly acquired Grado PS1000, and a Sennheiser HD820 on loan.
I've had an on/off relationship with the HD650 for almost 20 years. It's pleasant and well balanced, though a bit boring. Despite its warm signature, the midrange has a slight dryness, alleviated only by pairing with a vinyl source. Not near "top echelon" in any category, but quite nice overall when driven balanced with a great vinyl source (which I'm not doing, presently). But even for just a desktop setup, I desire more than HD650 these days.
The PS1000 sounded worse than the HD650 when fitted with stock "G-cush" (MEGA-bowl) pads. Simply reversing these pads (so they work like deeper flat pads) made a HUGE difference! Now they're decidedly bass heavy, but not obnoxiously so. Fun! Has that trademark Grado "snappy" sound. Very good midrange. A vastly more engaging listen than HD650, now. I really enjoyed my 1st week with these cans. I dig the look of its polished metal cups, too!
The HD820 is quite competent, but also reminds me exactly why I never liked HD800. I hated that super dry & austere midrange. Treble is out there partying on its own (and no one else is invited). Bass is a bit recessed for my preference. I don't get WOW'd by its detail & speed like I do with Stax 009 or even the old Qualia - not even close. So in the end, these cans (and HD800) have nothing going on for me.
Then the K340. Ah, what a beautiful midrange! Especially when paired to an OTL tube amp (like my SDS). These are 35 year old cans with both electret and dynamic drivers in each cup - so it may take a little time for your brain to adjust to some of the weirdness in their sound, but once it does, oh my! These cans responded beautifully to the nuances of the great old USA 6SN7 tubes. Surprisingly great detail. The treble is actually quite clean crisp and lovely, without being fatiguing. Bass is not cutting edge, but quite competent and well integrated - it doesn't detract from the music like say a Sony MDR-R10, or a poorly amped Stax. Because of its midrange, instruments and vocalists seem more lifelike inside your head, than with the other cans.
I'm also using the Headroom Max Balanced (great amp!) as a preamp in this setup, since I needed a Balanced-to-SE conversion to use the tube amp with my XLR-only DAC