i know this is old, and i bought one of these headphone Amps, and i did all the mods, panasonic caps, metal film resistors, B brown opamp. And i wasnt impressed for a year............until. i finally took away the Tone pot.
Why i didnt do this a while ago, i dont know, i always thought that it sounded kind of Muffly toned, couldnt understand the fuss. but removing the pot was a revelation, sounds fantastic. I feel no need to get an expensive Headphone Amp now, and it works very well, with my Sennheiser HD580's
I highly reccomend to do the mods and remove the tone

pot