For the best dynamic range from the head amplifier it is best to feed the amplifier with 12V via an unamplified Bias T. The base unit running off 5 V USB also has an additional amplifier so this may be over-driving your receiver.
If you are using a 1m diameter loop it will be resonant as a full wave quad loop in the VHF FM band. Make the loop larger or smaller to move the resonant frequency.
