LC Filter / Matching - Why does this cap help?
Your cap C6 closes the LC loop and becomes an impedance transformer with high Q LC filter with 44 dB gain or Q > 200 but then attenuated by 1k5 to -21dB loss.

I bypassed 1k5 to measure the Bode Plot. 

If you followed my hint to use Falstad's filter analyzer, then you can make this with ideal switch and caps to compare with/without.



from +44 dB gain to -21dB loss.