The negative numbers are "relative to" a reference level... 0.
Like, some set up their receivers so when the volume control is set to 0, the program audio will be at, say, 85dB SPL at the listening position... thus, when the receiver reads -20, sound pressure levels at the listening position would be down 20dB from the reference of 85dB, or 65dB.
Of course, that assumes all program material is recorded at the same levels, all equipment outputs the same line levels, etc...
As for the sub stuff... set your sub to approximately the same levels as your other speakers (taking into account the lowered sensitivity of most SPL meters in the low range) and it will be balanced with your system... and then if you're most people, throw away that setting and turn the sub up until your junk jiggles during either the famous "depth charge" or "tank tap" scenes in various movies.
Bren R.
