Treble bleed frequency cutoff
The amp is 0 Ohms and the speaker may be neglected.
But the ESR of the capacitor must be &lt;0.1 Ohm which rules 99% of electrolytics, so a metal filter cap from a microwave oven or store must be used.

In theory, the cap is not a LPF but a 4kHz HPF
The -20dB pad rises to -17 dB at 4kHz
Simply use 7.2 with C to compute breakpoint.
But to make a LPF is harder with a pad as R becomes 0.89 and needs a cap 8x bigger as a shunt.