Because all active bonus and malus never stack together, things like Duality of Mortal Presence (+10 int) and Enlightened Agony (smart) dont give +15 int but +10. Miasma of Dual-Mindedness (-10 int, per and res) and correspondant affliction give also -10 for the stat.
But equipment and weapon's bonus and malus are passives (like Echoing Horror as pointed by @Elric Galad) and stack with active bonus (but not with the same active bonus).
For example, Blind an enemy with Chilling Grave (grave calling) while he is Miasmed make him loose -15 Per (-15acc and -30 reflex). In other hand, Chill Fog instead of Chilling Grave and Miasma make him loose -10 Per (actives dont stack).
Tenacious from Resounding Call and Helwalker +10 might give +15 might, but only +10 might with Thunderous Blows.( see in first comment)
Chanter's invocation Each Kill Fed His Fury stack in the same way Duality of Mortal Presence etc since it is not Inspiration but only +5stats.
In picture, with a hired druid with every tiers 3 inspirations from the Least Unstable Coil, under the Each Kill... effect, and with Engoliero do Espirs (Ravenous Soul Devourer):
so +13 might, +13con, +8dex, +5per, +5int, +10res + other bonus from equipment (out of fight : Each Kill and Least Unstable Coil prolonged, Soul Devourer have no limit of time).
Here, the 75 fortitude of Birta is reduced by The Long Night (chant are not passive) for -10 (weakened) and -4 (-2might), and by Daze from Spearcaster (-10). With regular Daze (I mean by spell or ability) instead of from forceful Impact, the -2might would have been overrided by the affliction.
That work also with, for example, the Keeper of the Flame aroma (-8will from weapon) and Arkemyr's Dazzling Lights (-10will). Combined with Barb of Condemnation (-10 all defenses*), Miasma (-10int, -10res>> -40will), Ryngrim's Enervating Terror from the Whitewitch's Mask (Terrified, -10will) , Sweep you off (Street Sweeper, confused >-10will) and on top Ngati's Tusk aura (Hunter of Hunter, -4 all defenses, up to max at -12 or -13, so env.10), the enemy Will is now decreased by 98. It is more with Retentless (The willbreaker, -3will on hit with 5 stack/weapon).
*-10 all defense is not the same effect than -10 will, they stack. Same for bonus (Circle of Protection +15 all defenses stack with Shields for the Faithful +30deflection).
This topic is made because stacking rules are not really explicit in-game, and otherwise they are easy to understand with some practice.