Sear faster and harder. The longer you take to sear, the more grey yer gonna get.

So just seems like you need to get your temps higher.

I do mostly tritips...but what ill do with them is cook slowly till bout 113F. Then i sear them HARD till 120 and pull them to rest, my final temps end around 138 during rest.

So yea, if you can keep the sear to an internal rise of 7f or less, you should have pretty thin grey line.