My experience with the Helix with the 4 cable method is as follows:
Guitar in helix
Guitar out to Mesa Express plus 5:50 guitar input
Send out from Helix to return on Mesa
Send out on Mesa to return on Helix
This allows you to assign effect before the amp And other effects after preamp
I have not xperinced anything but total transparency and the best sound from any processor including Kemper.
There is no diminished or attenuation of sound