Just putting a bit of knowledge out there, since I spent a little while testing these today. It's a quirk I've kind of knew about for a while now, but hadn't taken the time to really explore.
Short Version. Core powers with damage multipliers stack additively with Spider-Sense, making them less effective with it than others.
Long Version. Rogue has several powers that gain bonus damage under certain conditions. Accursed Beam does extra damage based on your Max Health, Fangs of Farallah builds bonus stacks when you use cooldown powers, etc. These are multipliers and are applied to the power's final damage after other "base damage" bonuses like Damage Rating are applied. Spider-Sense gives all core powers one of these damage multipliers, based on how much Dodge Rating you've stacked.
The quirk that's not immediately obvious to most users is that these in-power multipliers stack additively with each other. When Fangs of Farallah has two stacks of +20% damage, it becomes +40% -- not a +44% like you'd get if they multiplied together. That much is generally understood. But then you have a power like Electromagnetic Flux, which adds +40% if your target has a DoT on them and +40% after holding the power down for 1s. These too add together for +80%.
Spider-Sense works on the same premise and adds its multiplier to other power multipliers before using it as one big multiplier. If you have Fangs of Farallah built up to +100% damage from cooldown stacks and then have a +40% bonus from Spider-Sense, the result is +140% damage. Therefore, Spider-Sense is ultimately half as effective on Fangs of Farallah compared to a power without a built in multiplier.
This affects the following powers:
- Accursed Beam. The bonus damage from Max Health is additive with Spider-Sense.
- Crushing Blows. It's sad to see this one affected because it didn't need yet another reason to be ignored, but yup. That +40% Boss damage stacks additively with Spider-Sense.
- Electromagnetic Flux. All of these bonuses are additive. The plus 40% from your target having a DoT and the two stacks of +40% that build up from holding it all accumulate to +120%. Spider-Sense's bonus is added to that amount.
- Empowered Shockwave. The +50% from having a toggle on is additive with Spider-Sense. Being the only cooldown power affected by Spider-Sense at all, it's probably still a win.
- Fangs of Farallah. Its five stacks of +20% add together for up to +100% damage. Spider-Sense is then added to it before multiplying the end total.
- Fantastic Funnel. The tooltip doesn't change to reflect multipliers (it used to be all DoTs did that, so not a surprise), but it was easy enough to test. If you have Human Torch and Invisible Woman's powers, the +25% is additive with Spider-Sense's bonus.
- Molecular Transport. I hit a snag testing this, as it seems there's a bug counting the stacks as one less than they actually are. I'll be off to report that here soon, but once I realize that it was very clear the explosive +200% per stack was adding with Spider-Sense, making for almost no significant damage increase to that component of the power.
At any rate, that's a less than obvious quirk to have in mind when planning your next build. Keep in mind that Spider-Sense does still work on these powers, the returns just aren't as good as it is on others.