Do passive RC filters need ground?
It needs to be able to return current to the source. This is to form a complete circuit for current from the source to flow around.

That return is most often ground.

It could conceivably also be the positive supply rail, provided the positive supply is well-bypassed to ground near the source circuit.

If your "overdrive" circuit has low output impedance and shares ground with the buffer circuit, that will most likely also work.