PreAmp Gain and Impact on Amp

My question is why when I adjust my gain settings on the Mark Levinson 326S preamp that are selectable per input of 0 dB 6, 12, & 18 dB the sound changes on each setting, biggest this is bass, and detail change, the higher you go the more bass you get to the point the system can sound overly warm. Amp is 29db at 8 ohms, 26db at 4 ohms, speakers are 92 Db efficient. 

Adjustable gain is what first pointed me to Aric Audio preamps. At this point,I really don’t want to be without. I rotate different amps,and I love the feature. 

What I can say about gain is the improvement it can make to your system. I have an ML amp and 326S preamp, and I can set the preamp to o gain, 6db, 12db, and 18db. Each step up from zero improves the sound, I like the 12db, but I recently tried the 18Db, and what I have experienced is a more relaxed sound, more open, more dynamics in the bottom end, and just a more pleasant and natural sound without any downsides. 12Bd sounds more sharper and detailed but not as natural as the 18db, So gain does make a sonic impact which is why buying a new preamp with more gain than you had before will sound different for sure and most likely better. My vocals are now in the room with body and openness. I guess much less like a recording.