dB is relative. +6db is a higher level, and -6db is a lower level. 0db is the "reference" level, against which all other levels are measured.
Now which level is 0db is completely up to whoever makes the scale. As mentioned above, 0db on mixers is usually the "optimum" level.
0db on other stuff could be another level. For example, gear is often said to be -10db or +4db. This is information on what the optimum level of the inputs and outputs of the gear is. They use other reference levels, obviously.
Since levels are all relative like this, 8bit and 24bit audio does not really have the same maximum levels. Neither do they have different maximum levels. :-) It's just numbers, representing the availiable levels from 0 to maximum, with different amount of steps inbetween.
The "maximum level" doesn't mean anything more than "hey, go any higher and you get distortion". What this level is depends on other variables.
If you aren't more confused yet I can make you.
Random Pavarotti Disease Victim.