You could up C1, 9, and 10. I'm not sure it will make a huge difference, as those aren't really "small" value caps, i.e., I don't suspect they are cutting a lot of lows relative to the spectrum of the human ear. But it couldn't hurt. So yes, you could try a 1uf film cap. .47uF or .33uF might be better though. Too big of a cap, especially at C1 might make it sound too "loose". You could also try increasing the value of C5 and C11. This might actually be more effective. Increasing C1, 9, and 10 will allow more low frequencies to pass. Increasing the emitter bypass capacitors at C5 and C11 will amplify signal at a lower frequency.
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