I ran some simple caluclations to give an example of what you might want to try and what you might want to avoid. You would want to tune the box in the range of the two flattest curves (Red +1 db bump and Black (nominally flat) with a +0.1 dB hump)
You want to avoid tuning the box too high as shown by the green curve. This will result in a +3 dB hump in the bass curve centered around 45 Hz. You will get more bass, but the transient response will be degraded.
These are approximations of what you might get in a dead medium to large room. With an actual subwoofer you would have to include room effects.
