Bard Instrument Modification?
 

Can somone explain it to me? Is it 1% per modification?

The vendor drums says instrument modification 20, when the mistmore battle drums says 21. What does that mean? 20% or 21%? Sorry, very new to the bard and I want to learn while leveling

It means the effects are multiplied by that number, so 2.1 is effectively +110%.

Keep in mind that the really important stuff (mana regen, slow%, haste%, snare%) isn't affected by that multiplier. Damage, regen, and resists are though so they are still situationally useful.

In theory, wouldn't the bard epic boost Cassindra's Chorus of Clarity (level 32), since it's singing? Also, since Cantata of Replenishment (level 55) is string, wouldn't that mana be boosted by an instrument, or is it only modifying the healing portion?

The mana regeneration portions are just not affected by instrument modifiers.