Angstrom 200 preamp

I just bought an Angstrom 200 Home Entertainment Director for use as a stereo preamp which has controls that I've not encountered in my 50+ years in this hobby.  Ive searched the internet for an owner manual and discussed the issue with others generally more knowledgeable but still have questions.  There is a master volume control which seems pretty straightforward in that it would presumably be the same as a control simply marked as "volume" but also includes 2 led's which indicate max and min settings and which remain unlit with settings in between these extremes.  It also, though, has an input level control with 3 settings, -3, -6 and max dB which I have no idea how to use. I did come across one review by someone who also had no manual who said that he used setting levels just below where clipping occurs but that idea makes me nervous. Anyway, I'm asking for some guidance. 

How i I got here is this; I was using a Cary Rocket 88 to and old Theta Dac to Serie Reference 3a MMC speakers but bought back my pair of ML Ethos speakers along with an Ayer V3 amp and am very satisfied with the sound. In a trade I also acquired this Angstrom 200 preamp which I tried in my garage setup (old Russian tube amp and B&W CDM speakers). I'm not very good at discerning subtle changes but the crystal clarity of the resulting addition was incredible.  This test was done using analog signal from a laptop compared with digital out through the Dragonfly DAC.