As @youngho mentioned, the major issue with the front wall is SBIR -- speaker-boundary interference response, which causes cancellation at a frequency whose quarter wavelength is the distance from woofer to boundary. For that reason, it's good to keep the distances different from the various boundaries (side, front, ceiling, floor). The first step is to make sure the distance from woofer to front wall is different from the distance from woofer to side wall.