I made a set using just resisters and I couldn't hear any difference (used that Uneeda/pads page as a guide.) There is a little bit of science involved in choosing the right resister values so you match impedance of input and output, but it's not rocket science, and I expect anyone in that business would know how to do it. I can envision someone using impedance matching transformers, and that inductive load might change the sound?
I made balanced XLR > mini TRS so I could run my V3 analog > Korg MR1. This had 20db drop and allows me to have the V3 meters match the Korg meters. I have run V3 with analog going into these cables > MR1, and at the same time the digi out of the V3 > R4. Then I made a wave file with alternating one source, then the next etc, and I couldn't hear a difference. What I was trying to test was V3 A/D vs. MR1 A/D, but again... I couldn't hear a difference. Are my ears not fussy enough? Perhaps.
My suggestion is look at make your own as described on "uneeda", or try Niaint (spelling??). I have a set of 15db XLR attenuators from him you can borrow for a test if you like.