are you using flyhalf's twrp? http://forum.xda-developers.com/desire-601/orig-development/recovery-twrp-2-8-3-0-touch-recovery-t2991449/
I did it while on InsertCoin, so yes, it works on KK.
I'm not sure if it works with stock, bc when I tried it with stock I happened to have avast running and it didn't work.
I tried it with IC, this time I decided to disable avast and it worked....
OK, the HTC Site says that the 601 WON'T get Android L. awwww
"Virgin Mobile HTC Desire 601
This device is currently on KitKat, and will not be upgraded to the Android Lollipop Release."