Thanks for the very helpful answers. I did rather think that an isolating transformer was likely the only way out of this and the Radial Tonebone looks to be most cost effective option. Still have slight concern about so called "tonesuck" but I guess if I buy one from the Amazon monster I can send it back if I detect any which does seem a little unlikely based on the comments here. It is clearly designed for this purpose and hopefully impedance matching or any passive attenuation aren't going to be evident.
Not at all sure why the KPA is more susceptible than the Fractal or Helix - as you say may be something built in to those devices.
As an aside I am impressed with the KPA. Haven't dare launch yet another comparison thread (tho suspect here there is less chance of starting a forum riot - kinda easy on Fractals place...!) but the profiled amps feel and sound a good deal more real than the other 2 competitors and I rather like the way the fx have been implemented. The interface (simulated amp) is OK but still feel the Helix suits me better.
Thanks again for the replies.