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.
