Let's just say I send out my swoobat with simple knowing stored power. Then a gyarados comes in and lowers swoobats attack 2 stages. So, will stored power's power be raised or lowered?
Stored Power deals variable damage depending on the positive stat stages of the user. When the user has no positive stat stages, Stored Power's base power is 20. Its power increases by 20 for each positive stat stage the user has, and does not decrease in power due to negative stat changes. It reaches a maximum power of 860 [20 (base power) + {20 x 7 (Attack, Defense, Special Attack, Special Defense, Speed, evasion, and accuracy) x 6 (maximum positive stat stage)}].
Bulbapedia Stored Power Page
Stored Power's base power will not decrease, but it will not increase either, as it states here, Stored Power's base power goes up every time the Pokemon gets a stat increase, not a stat decrease.
Source: The link above and logic
