I just found an interesting link on the 360 Systems 20/20 Frequency Shifter:
It gives a nice overview of how frequency shifters were used in some classic electronic music.
Interestingly enough, the color scheme of the 360 Systems unit is very close to the ValhallaFreqEcho colors:
This is pure coincidence, as I just found this post on Matrixsynth. The 360 Systems Frequency Shifter is using Futura for its font, which I used for awhile before switching to the grotesk fonts I am currently using.