The trim pot network resistance total is about 4K, not including the unknown power amp load, so a 0.1uF cap alone will block all the bass, ie everything below 227Hz. ( f=1/(2piRC) ) We need to know if the input has any DC and what is the output load impedance. If there is no DC from the preamp, then any capacitor is redundant, and you would be better with a jumper short instead. If there is DC then you need to know the polarity so that a polar cap can be oriented accordingly. 100uF (0.23Hz) is probably overkill, 1uF (23Hz) is minimal and 10uF (2.3Hz) a good choice. The 0.1uF is there because large caps can be inductive, but it is unlikely it actually makes any difference. Given that this is the bass channel, a high frequency bypass is completely useless.
