What ever effects unit You like, but the problem is You can only add it on the master out, as there is no send/return for external fx. I use Korg Kaosspad 3, also cool effect units are rmx1000, Boss sp404, Dreadbox Hypnosis, RMX500, Older pioneer efx1000, Korg mini kaospad and so on… You can also add some effect pedals like Headrush or Boss too. Just check what You need and what works the best for you.