What happens if I change the value of a coupling capacitor in a tube pre-amp
Yes, reducing the coupling capacitor will reduce the bass-response. 

Technically, the capacitor and the output impedance of the tube-stage forms a simple high-pass filter.

Your circuit is very typical: You have the 100k plate resistor. Along with the internal resistance of the triode, that roughly gives an output impedance of 50k Ohm.

Together with the 0.022µF capacitor, you have a cut-off frequency of 1 / (2 * pi * r * C). With your values, that'll give a cut-off at 144Hz.

If you want to reduce the bass response, you can lower the capacitor; however, it is likely much more musical to roll off the bass using a high-shelf filter in front of the amp, and tune it to your liking. The difference is that the high-pass will remove the lowest frequencies, while a shelf filter just attenuates them.