1.) An extra bias voltage is necessary because you are working with single supply only. However, I would suggest to use a split supply (if available) without such external biasing.
2.) For lowpass stages, the resistor between the first node and ground can always be set to infinity (can be removed). This removal is not necessary but this resistor complicates the dimensioning.
3.) The Sallen-Key topology is very sensitive to tolerances of the gain setting resistors. Therefore, it is recommended to use a design strategy based on unity gain values or gain of two (two equal resistors in the feedback path, any values). For this purpose, there are several filter design programs available (online or downloadable).
4.) For a 4-pole filter (as shown in the figure) you need two stages with DIFFERENT pole locations (that means: no identical stages). The parts values, of course, depend on the desired cut-off frequency and the selected approximatioin (Butterworth, Chebyshev,...). Use filter design programs for finding the values.
5.) As it seems, the online program (OKAWA, your link) works for a second-order filter only. Because you need two different stages, you need two different pole frequencies with different Q values. If required, I can give you the values (based on the specification as mentioned under 4.).
UPDATE: As mentioned under 1.) the dc bias of +5V is necessary because the negative supply pin of the opamps is at ground. It is best to use a symmetrical +/-12 volts supply for the opamps. In this case, of course the +5V are not required. Connect this resistor simply to ground. This resistor is necessary to allow a small dc bias current for the opamp input (the value may be larger if you need a larger input resistance of the whole circuit.)
The TL081 is a low power amplifier, not meant to drive headphones. The roughly 600ohm R2 does, or can do, a number of things, depending on what this circuit was intended for.
1) Protects the output from excessive currents if low impedance headphones are plugged in. The amplifier itself is already protected against short term overload, but not continuous overload.
2) Increases the output impedance into a more standard 600 ohms, if it's intended to be used with other audio gear.
3) Decouples the amplifier output from the capacitance of long co-axial lines. This is important for stability.
4) If the amplifier were a different higher power one, then R2 would attenuate the signal down into headphones.
R1 appears to be a 'belt and braces' sort of component. It doesn't do any harm. It does provide a DC path across the output, whether the amplifier is powered or not. If a decoupling capacitor were provided between amplifier and output, then it would indeed be useful to maintain 0v at the output.
Best Answer
Yes, you can do this with little or no side effects. The resistor is not needed for your application because you are using an opamp buffer to drive the shared line. Just ensure the output of the buffer is nominally centred between the supply voltage rails.