using a XR1071 as a LPF/HPF/VCA
It would work as a: -


LPF - bass booster (but not bass cutter)
HPF - treble booster (but not treble cutter)
Voltage controlled amplifier


What it wouldn't do (and this is as important for a good synth as nearly anything) is band pass filter - synths use these a lot - in fact I would say it's fundamental to synths - without being able to sweep thru a range of the spectrum with a BPF (peaking filter), it's like having a car without wheels.