Harmony algo

The Diatonic algorithm is what you want.  Two additional harmonies, with both intervals constrained to scale & key.  Plus a full featured dual delay, with some unique feedback routing.  QuadraVox will give you four additional voices in scale & key.  Both have the same maximum delay time, with QV carving that time into rhythmic patterns [no feedback].  Either one can enhance the doubling illusion with short delays & panning FX.

Keep in mind that for some of those classic songs, you’re going to need a way to change the harmony intervals, or even the key / scale at certain ponts in the song.  An Aux switch, expression pedal, or MIDI can make those transitions easier, but switching between complementary presets would work.

Diatonic is the working man’s pitch algorithm. Two voice ‘intelligent’ harmonizer, any key, 13 scales.  Pristine dual digital delays; 0 ms. to about 1.5 seconds.  Zero to infinite feedback, and a unique, musical routing based on the longer delay time.  PITCH MIX is in a position that allows delay trails and looping.

Four harmonies can get a little thick in a live scenario.  Sometimes that can be a good thing [Hammond B3 from Hell; jazzy synth, or massive power chords from one or two notes; riff rock layered lines].  I don’t think that the pitch tracking suffers from adding 2 more voices.  Like any pitch shifter, the formants can get a little ‘cartoon-y’ at the extremes.

Keep QuadraVox on your radar, though.  The Delay Grouping can be amazing under pedal control.  Scale-accurate arpeggios & ‘fingerpicking’, slapback 4 voice chords, and every delay rhythm in-between; all under foot control.  More for originals and experimentation than a bread ‘n’ butter covers set.  One practical technique is to take two harmonies, clone them, then spread those two doubled harmonies with short delay spacing and panning.  Powerful stuff.