Mark Levinson 326s

I have kept my 326S set at 0db for inputs 1-3 which I believe is the factory default setting. I listen to mostly jazz at medium volumes and at this setting I can keep my volume control between 4 and 5 listening to either my ML512 or McIntosh MR88 satellite tuner.

I own the 326S as well and I can tell you the sonics will change based on the gain settings. 0-6-12-18db are selectable. Going to my McIntosh MC 402, 9 gain sounds thin and lacks dynamics 6db of gain brings out more body and dynamics, and bottom end, going to 12db of gain really adds much more of what the 6db setting does, 18db of gain is overripe and to warm. So they all make a sonic impact, so try them, I stay with 6db of gain but at times I like the 12db of gain, the 0 & 18 dB of gain sound the worst. 

Update. I purchased the matching ML 532H amp. 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.