ASTM A304-05e2 Standard Specification for Carbon and Alloy Steel Bars Subject to End-Quench Hardenability Requirements.
If you have used ER 70S-6 it will most likely weld just fine(lack of cracking being the criteria I'm speaking of), but these are high hardenability materials intended for high strength applications, and the S-6 will not match the Quench properties of the low alloy material either with or without a quench. In fact, quench won't get you much with S-6.
I'm guessing here, but I would say that you may have an undermatched weld no matter what filler you use, depending upon the specific alloy of course.
This, in my opinion, is screamin for engineering and testing.