In addition to my prior post, if adding a subwoofer to a preamp that even has a subwoofer output, with no impedance changes to play games with the signal, still can degrade the sound.
The bass from a sub could mask the midrange and highs easily. Even more so, if you don't set it up right. So do some serious comparisons with, and without it (during and after setup), to see if hear anything that can sound like it is taken away. This can be due to the sub possibly masking, or overpowering, the good part that you are hearing, in comparison to not using it. Other distortions from the added sub, or bass vibration in the room, can exist too.
The bass from a sub could mask the midrange and highs easily. Even more so, if you don't set it up right. So do some serious comparisons with, and without it (during and after setup), to see if hear anything that can sound like it is taken away. This can be due to the sub possibly masking, or overpowering, the good part that you are hearing, in comparison to not using it. Other distortions from the added sub, or bass vibration in the room, can exist too.