A mini-review and comparison with the photo:
In my post of Randy's Revenge in this thread, I sang its praises. As of today it continues to earn my respect. My recent acquisition of Feber practically demanded that I compare the two pedals directly. So that's what I did.
Tested with bass guitar, the results were unsurprising. Randy's Revenge is per Fairfield Circuitry "all analogue". Feber is at least mostly not analog, being based on the Spin FV-1 chip. Whether because of that difference and/or any other, the audible difference between the two pedals was distinct, and while playing it was palpable.
The functionality of Randy's Revenge largely is a subset of that of Feber. Comparing them with apparently equivalent settings ("apparently" since I can't know whether, for example, the Mix control of each is linear or what kind of mixing it is doing), the frequency response of Randy's Revenge went both lower (deep fundamental) and higher (present, tactile highs), but did not sound scooped. Feber tended to sound tubby and feel distant by comparison. This was truest when the modulator was at audio rates (for classic ring modulation) rather than at LFO rates (for tremolo).
So basically, to my taste Randy's Revenge simply sounded better than Feber.
For tremolo Randy's Revenge sounded "purer" wherein I heard only amplitude being modulated; whereas at the equivalent settings Feber seemed to be doing some slight frequency modulation, so its tremolo always hinted at harmonic tremolo.
But that's where the comparison ended. Feber's 'single sideband' and feedback functions ranged into sonic territory where Randy's Revenge just can't go. Especially the low sideband, fully wet or mixed, was revelational for bell timbres, played with chords or single notes, cleaning up the high inharmonic stuff as it did. Really cool.
At LFO rates, Feber's feedback gets into phasing that can list to flanging-like "tubular" phasing. I was reminded of the mighty Haible Tau "Pipe" Phaser, which I own as a eurorack module. Impressive.
Feber by Pladask Elektrisk