i have the iesl since some weeks and love it. i like to build my amps myself from time to time, love the desings from nelson pass. when i start to build nelsons sony vfet amplifier i had the plan to add a voltage divider to drive my dynamic headphones. and right at this moment the iesl was available so i ordered one. would be great to listen to these great amps with my headphones. now i use the iesl most of the time together with the sony vfet amp and the stax sr-007. the iesl is very neutral energizer which transports the sound of the amp who is connected to your headphone. having the beautiful sony vfet sound with the sr-007 is magic for me, the frequency responce if the iesl is quite linear, deep bass up to the mids are perfect, the highs are a tad extenuated. so the iesl is a non sounding device with the ability to transport the sound of the amp. the limits of the iesl is the resolution but it depends on the headphone. when i compare the sr-007 out of the iesl (via the vfet amp) with the blue hawai se the resolution is quite similar, maybe a tiny bit in favor of the bhse. nothing i would miss. with the sr-009 it's different, out of the iesl you are loosing some details, responce and soundstage is perfect but you miss the last details in contrast to the bhse. these top perfoming headphones like the sr-009 or the focal utopia are not allways easy, with the sr-007 i can enjoy all my music. with the utopias an even more with the sr-009 out of the bhse a part of my music is simpy to bad recorded or mastered to enjoy. the tiny loss of resolution is similar to great tube amps (luxman) with output transformers, the effect is a small softening of the sound (and dont think at cheap tube amps). with dynamic headphones it's tricky, when i use a voltage divider adapter i made out of my power amps in contrast to the iesl i prefer the voltage divider. the iesl softens the sound quite a bit more then with electrostatic headephones (this effect is more audible with a easy to drive utopia, a lcd-3 sounds better out of the iesl). also the control of the bass get's a bit loose trough the iesl. with sensitive dynamic headphones you hear also the noise floor of the amp. the differences between a good voltage divider and the iesl are not enormous but audible. the iesl needs power and control to sound good. my 3.5 watt class a dynalo is to weak, the dynahi with 15 watt class a is perfect. the sony vfet with 20 watt stunnig. the sound quality i get out the sony vfet / iesl combo beats a stax srm-727 easy. the iesl is capable and scales up with the amp you are using. i think only a few headphones amp are potent enough for the iesl. but if you have a nice (class a) amp with around 20-50 watt and electrostatic headphones the iesl is a great joy to use. it's not a phrotesis in contrast to a standalone electrostatic amp but a great possibility to get a better match of the audio chain you are listening to.