Yamaha THR100HD as power amp for KPA / Noise in effect loop?

Hi folks,

I am currently checking out an interesting solution for the KPA as a) a power amp b) as a backup. It is the Yamaha THR100HD that in my opinion provides a very natural tube sound and comes with a stereo power amp. I using it with a cab driving an Eminence SWAMP THANG and it sounds really amazing.

The KPA sound also amazing through the effect loop return of the Yamaha. However, although the Yamaha as such is very low noise, engaging of the effect loops unfortunately provides an unpleasant hiss. Too much for my taste...

Tried anybody else here the same combination of Yamaha THR100HD and KPA? Could it be that I just got a faulty device?

Thanks and cheers
Sacapuntas