I bought both (HM-601 and a HeadStage Arrow).
I'm planning on using the line out on the 601's to the Arrow.
Does this sound like a good combination?
I have the 601 and the Arrow HE 1G.
They work very well together. I use the Arrow with my RE-0 for portable and a Little Dot Mk 1+ with my MS-1i for at home less portable.
I have been trying the Arrow out with my Koss KSC-75 with a P to S adapter with success and it also works very well with the MS-1i but I have always liked the Little Dot - MS-1i combo so I keep that for not portable.
While I like the HO on the 601 and it is great for doing stuff listening I find that an amp just gives it a bit more of everything for real listening if you know what I mean.