On this occasion, I agree with jade falcon, there are two versions of the k340 (I have both, and a k4 and k145, so all the electret/dynamic cans that akg produced). However since the drivers and passive radiators in the k340 are the same as the sextets there is a chance a k340 could be modded with dynamic drivers from one of the sextet varieties. I attempted to mod a previous pair I had with grey drivers an red passive radiators, but they still sounded a tad anaemic to me. The bass was tight, albeit a bit thin. The k340 has some of the smoothest mids and highs you can get and an awesome sound stage, but yes bad seals and zero head clamp give little consideration to the back wave which could be it's downfall in the low end.
Never had a problem driving any of them with a fiio e17 tbh, no need to go to speaker taps o.0. In fact while we are on the subject of tapping straight into speakers output there is only one non electret that I have ever had to do this with and that's the wharfedale isodynamic (check out the vital statistics for those cans!) Categorically the best headphones I have ever heard (and I have approx 70 pairs of vintage cans, about half are planars... if only I had that many pairs of ears o.0) although not auditioned audezes 🙁 which incidentally were originally conceived when Alex what's his face listened to some wharfies 😀. The wharfies can be driven by the e17 too and have a slightly elevated low end (some of the only planars I have heard with bass, and not bass bloat, pure tight bass as it is in nature and they go sub (I can hear down to about 17 hz, and they deliver). The wharfies have been on my head pretty much every day in the office since 2013, bloody comfortable too, sound as good as they did in 1974, the membrane is made from the same stuff they did the electronic traces on in the space shuttle.
Dynamics are good in'all but once you hear planar you don't go back IMO. Not a stax fan either, again pure highs, but I struggle to listen to stuff when the bottom half of the range is non existent. I like dnb 😀
Headphone collectors are no different from collectors here and caused the prices of a lot of vintage planar headphones to skyrocket about 4 years ago (you can blame the "orhthodynamic" and "show us your vintage cans" threads on head-fi for that!), a k340 is almost the equivalent of a voodoo5, they come up every now an then but you will be paying more than 100 quid at least for them in any condition... once I found the wharfies I got out of that game so stopped following 'vintage headphones' in my ebay search a number of years ago. Gawd knows what the prices are like these days o.0