Further to PWM DAC Explanation
From your previous question, it sounds like Dave does not recommend using that method.

There likely isn't a point in using an LC filter in line after the 4th order sallen-key. Depending on the quality of your sallen-key, you may not get any more benefit out of an LC filter.

With such a low sampling frequency, and no need for high output impedance, why aren't you maximizing the PWM frequency and filtering it with an appropriate RC filter?

From your datasheet:
"PWM Frequency for 11-bit resolution
(@ 30 MIPS) = 29.3 KHz for Edge-Aligned 
mode"

Your sample frequency is 40 hz which is 3 orders of magnitude different. A single RC filter gives you a single order filter which gives 20db/Dec. With 3 decades of drop-off, you should get a pretty smooth signal out relative to your sample rate.