About Vocal Harmony seems to go away, try if this help something:
First I consider that the notes added by the Vocal Harmony, are a little robotic.
The Vocal Harmony volume (Mixer and Auto Harmony On/Off buttons) and
digital effects DSP depth, are not volume type but Dry/Wet type, as a Balance.
This is, e.g. Voval Harmony volume: 1 - we only hear the lead voice; 127 - we only hear the effect/ harmony notes added. So, 64 represent singer voice volume and
Vocal Harmony notes with the same volume. Less than 64 the singer voice is louder
than Harmony and more than 64 the Harmony is louder than the singer voice. For me
30 is a correct value for consider this feature of some use.
The block diagrams of PSR 8000 shows that, before the Vocal Harmony block there are two HP (high pass) filters to correct the mic signal. Te PSR 9000 has a 3 band parametric equalizer more a noise gate block and also a compressor to correct the mic signal before the Vocal Harmony block. The PSR 740 has no of this addicional blocks before the Vocal Harmony block; the mic attack directly the Vocal Harmony. Surely here it is the problem. So, if we use an external mix console with 3 band EQ, gate noise and compresser to correct the mic signal and after we inject this corrected signal to 740, I guess that Vocal Harmony at PSR 740 will work perfectly.