Yup...solely because of K-On! I've considered buying a pair of K701's, but been able to curb that compulsive fanboy urge so far. 
As for best headphone for listening to them, krmathis is right that it really depends. Personally, I would think twice about using phones that you consider "bright", but I also find that kind of sound signature fatiguing. Some of the songs from K-On! seem to be mixed a little hot, IMO, so I think a Grado would put me in pain. Like Utauyo!! Miracle has a lot of guitar noise and treble in it, not to mention Yui's voice is not what I consider "smooth". Some folks might really like the brightness though, so maybe for them a Grado would work fantastically.
I use Yuin PK1's at work and usually LCD-2 at home if I'm listening to K-On!...or anything else for that matter.