That's normal with pretty much any headphone amp, and it's due to not being grounded. For all the shielding to work in your bass and pedals they need to be grounded. If you plug one of the outputs into an amp it should make the noise go away, or find some other way of grounding.
Thanks
It is still a bit of a surprise as my RB is pure silence while the SGT-DI buzzes some. I guess I should not be so hard on the SGT and be happily surprised with the RB’s headphone out?