My passive speakers sound bass-light, although the distance between the speakers and my listening position is the same. What is wrong?

Ensure proper polarity when connecting your speaker cables. The cables usually have a (colored) marking and/or printing on one of the two conductors, which indicates the polarity. Connect the marked conductor to the positive pole (usually marked red) on both the speaker and the amplifier. The connection scheme must be the same for both speakers; otherwise, you will experience the described sound image due to the inverse movement of the woofers. Also, check the sound settings of your amplifier.