Generally, Magnetic effect only consider minion's stats when it's played. Other H/A modifications it gains before it merges is never considered, including on-play effects (Hobgoblin) and auras (Stormwind Champion).
But in Dalaran Heist, Haro Setting-Sun (Heroic)'s hero power Passive Hero Power: Your opponent's minions have -2 Attack does affect Magnetic minions. In this video, bomb has -2 attack, so if Zilliax gives it +3 attack it should be 1 attack. But actually it shows as 0 attack.
https://www.bilibili.com/video/av54071518 (3:30 to 3:45)
Generally, Magnetic effect only consider minion's stats when it's played. Other H/A modifications it gains before it merges is never considered, including on-play effects (Hobgoblin) and auras (Stormwind Champion).
But in Dalaran Heist, Haro Setting-Sun (Heroic)'s hero power Passive Hero Power: Your opponent's minions have -2 Attack does affect Magnetic minions. In this video, bomb has -2 attack, so if Zilliax gives it +3 attack it should be 1 attack. But actually it shows as 0 attack.