Hi,
You have your NFB resistor in the area of 14-25K, so now you should sub a fixed value for it.
There are six standard 5% values in that range... 15K, 16K, 18K, 20K, 22K and 24K. A local Radio Shack might carry of that, 15K and 22K (they still seem to think resistors are 20% tolerance, LOL!) I suggest you tack in each resistor and do a few critical listening songs you're very familiar with and permanently add in the one that sounds best to you.
If you want to know how much NFB you have, use an analog AC meter with a dB scale. Set to 0dBm with a 1KHz sine wave. Remove the resistor and see what it shoots up to... that's your dB of gNFB!
Cheers!