Hi all, I'm a newbie, this is my first post...
Mr Toft, +6 (dB) is strictly a level thing. I used to align these beasts back in the olden days, and am running on memory here, so am welcome to corrections...
Ampex created a standard for their then-popular 406 tape, which stated that 0dB on the meter would represent a "flux density" of 185nWb/m (or "nanoWebers/per meter, or how much the microscopic magnetic particles were "disturbed" on the tape, not unlike iron filings being moved about by a horseshoe magnet). Since that time, newer tape formulations (456, 496, etc.) came along that allowed for greater signal levels before the tape started to saturate, allowing you to record as much as +9dB hotter (depending on the tape), thereby effectively reducing your noise by that same amount.
To know what level your machine is set up to in regard to "flux density" or "nWb/m", look at your test tape box. If it says it's 185 nWb/m (Ampex operating level) and you thread it up, and your meters read 0dB when playing back the test tone, then you know the machine is set up to 185 nWb/m. If your meters show a -6dB, it's probably set up to 370 nWb/m (the inverse of the chart below). Setting your machine up to +6dB means calibrating your machine so that when it says "0dB" on the VU meter, it's actually recording 6dB hotter than 185 nWb/m or Ampex operating level. When used as a reference, Ampex operating level defines these other levels:
0dB = 185 nWb/m
+3dB = 250 nWb/m
+6dB = 370 nWb/m
+9dB = 520 nWb/m
Setting up bias is a whole different ball game. Bias, by the way, is only engaged when recording, not when playing back.
Regarding noise, typically the narrower the tracks, the greater the hiss. This is why a 1-inch 8-track machine would have less hiss than a 1-inch 16-track machine, the latter often employing noise reduction techniques such as Dolby, DBX, or SR, or just blasting the tape with as much signal as possible, given the right formulation.
Hope this helps, and that I haven't belabored the point too much.
Van Nuys, CA