First off, on the damage dealing end (as damage receiving is a separate set of reactions/modifiers). There are 4 important main components:
1.) Base damage = the flat value that the hit will generate regardless of any other buffs present.
2.) Random damage range = with most physical hits and some magic hits there will be a random range attached, this can be pretty big for some bosses (XT is something around 10k it seems).
3.) Attack Power = I'm not 100% sure the factor is the same as for players, but I don't know why it would be different. AP is divided by 14 and added to the *dps* (not the damage range).
4.) Crit flag = if yes, double damage, if no, no bonus damage.
We can ignore #4 as a tank who should be uncrittable. For #3, bosses have a set AP value, I haven't looked into it a lot either, the reason being that it is generally expected with the saturation of buffs that someone will be maintaining demoralizing shout/roar or another similar AP debuff (hunter pets, lock curse, etc). It's not a variable, it's there or it isn't so it's not vital to understand the direct value, just know that it is a reduction in your damage taken and is often not at the expense of anything else (some classes it is, like lock curses). The dps is based on the average value of the damage range (i.e. midpoint between min and max). This is where your language is a little mushy. If the damage dealt is 2000 + (100 to 500) the average will be 2300, but there will not statistically be any more hits at 2300 than 2500, it's an equal chance of any of the random values. The random damage range is always present, and like the base damage, will not be affected by anything, only the AP component which is added on top of those values is significant. To find dps as opposed to damage, you would use the base swing speed (before modifiers such as haste/de-haste) with the average damage value. Divide AP by 14, and add it to that value. That will tell you how much AP is adding to it.
For some sample math:
Boss A = 10,000 base damage, random damage range 3000-7000, base swing speed is 2.0 seconds, AP is 2100
Average swing before AP is 15,000, speed is 2.0 sec, so dps is 7500 dps. AP divided by 14 is 150, added to the dps is 7650, that will be 15300 damage per swing.
If you have a shout/roar that reduces boss AP by 700 (random numbers for clean math), the boss's ap now only adds 100 to the dps instead of 150, so the final hit value will be 15200, so the debuff reduced the swing damage by 100.
I'm not sure what the actual AP values are, nor that the exchange is the same as it is for players, or applied the same way, but that IS the equation for players, for how what that may help.
I want to say I've seen charts for Boss AP values, but I can't seem to find them right now (silly crappy work computer).