To get the best balance of sounds in Box Of Stops, typically you will want to disable Local Control on your keyboard.
If Local Control is enabled, this means you will be hearing another sound, often a piano, generated by the keyboard for itself. Box Of Stops generally doesn't use the full channel volume available for each instrument, so the locally controlled channel may also dominate the sound.
Setting Local Control
This will depend on the individual keyboard.
Typically a higher end keyboard has a control panel, and Local Control will be a setting in such as the Midi menu on this.
Otherwise it is usually a case of taking a look the User Manual for a relatively arcane key sequences. For example with a Roland digital piano you must press Metronome+Reverb together and B7 or C8 to toggle local control off or on respectively. Similarly for a Yamaha Clavinova you press Demo+Piano together and then C6 toggles both ways.