Just replaced my old LinkSys router by the Asus RT-AC68U.
DD-WRT seems not to be available. Only a thread that describes how to flash it. Not sure when I will do the flashing.
http://www.amazon.de/gp/product/B00897IKNK/ref=as_li_ss_tl?ie=UTF8&tag=data0f-21&camp=1638&creative=19454&creativeASIN=B00897IKNK&linkCode=as2
Did you make any progress finding the DD-WRT ROM for you Asus RT-AC68U?
This forum has the link:
http://www.dd-wrt.com/phpBB2/viewtopic.php?t=177612&postdays=0&postorder=asc&highlight=rtac68u+build&start=345