IMO, after playing around with just one, you need to test it out at different ports on your network chain. I've tested it out at several ports, and at my WAN port (in) to my Asus router, and it sounded best there. I 've just added a second one, and the effect does compound, but i've kept it at the input to my FMC after the output of my Asus router. IMO system of course.
I tried it out at the Eth input of the ZenStream and i concur my findings to what's reported by others above.
I too ground my ZenStream and EtherRegen and noticed the same as you. Me too, I do find iFi's clean up products effect does compound, but placements and careful listening will be beneficial to hear where it really needs to be. Because of my network setup and my OCD to clean and isolate the signal chain within the network, i now have 9 ifi Ipower X's and 2 Lan iSilencers in the chain. I am a firm believer that network optimization and isolation are critical and a must. Once you hear this difference, there's no going back. Youd be pleasantly surprise how well your current source (in my case - ZenStream), can perform.