I've been listening the past two days to my new K1Ks through my trusty Sex (with Ni MQ iron upgrade, C4S and upgraded caps). While these cans supposedly need 300B power I have not found any hint that the Sex can't make them rock. I have to admit that for the first hours I only did some Baroque music but right now I'm feeding them some rock. I'm spinning The Cars self titled album on MoFi which, BTW, I strongly recommend and the sound is full with very nice punchy bass and the soundstage these headphones are famous for.
Maybe my listening volume is not that loud (I doubt it) or maybe the fact I usually leave my "sub" on when lsitening to headphones (the FP3 feeds the sub amps, the headphone amp and the speaker amps which I do turn off) help but I don't feel I'm missing much.
Of course I'll rebuild my 300B amps for these babies but just wanted to point out that Sex CAN drive K1Ks no problem. This is a truly wonderful amp that one could build a fantastic system around for high efficiency speakers and/or any headphone you could think about.