krank krankenstein fx send vs line out (with impulse)

Haven't listened yet, but which impulse did you use?
As far as I know, Redwirez impulses are the only ones to use a solid state poweramp.
Which means if you used a different impulse, the line-out (which is tapped from the speaker out) contains poweramp + preamp -> poweramp EQ curve from impulse. Generally, you would EQ differently in your DAW to compensate for this.