The only way to increase Combat EXP obtained from quests is by having the Chenga – Sherekhan Tome of Wisdom
Here is a guide on how to get the Tome.
Note: The following does NOT increase/buff your Combat EXP from quests in any way
- Pets Combat EXP +5%
- Server Combat EXP buffs (EXP Boost Event – yes, that includes Season server as well)
- Food Combat EXP
- (simply anything else other than Chenga Tome)
On the other hand, any other EXP given by NPCs such as Life EXP (Gathering, Trade, Cooking, etc) can be buffed by pets and food. But it may not be affected by server wide EXP events.