B1 Buffer Preamp

1uF should be fine. Are you sure its 1uF and not a fake - incorrectly labelled. If the cap is good and the pot is correctly wired you might have an impedance mismatch with your source. IE the frequency attenuation might be being caused by the volume control interacting with the output cap of your source.
 
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If everything is OK then it might be down to the rest of the set-up. The B1 is pretty transparent so wont affect the frequency response of the rest of the set-up. Try taking the B1 out of the system by simply connecting the wiper of the volume pot to the power amplifier.