August 2018 Version Update

Do you mean whether I have tried Enspells with Affinity gear or whether I have tried stacking Affinity with the Orpheus's Sash?

The latter. Same with elemental staffs since they are a different term

It stacks with both Chatoyant Staff and with Levante/Marin+1.

Base: 102 AE

Levante
No waist: 117
Orpheus (MD): 132

Marin +1
No waist: 113
Orpheus (MD): 128

Chatoyant
No waist: 117
Orpheus (MD): 134

So it's additive with Affinity and multiplicative with Elemental staves, which would indeed make the belt an Affinity bonus.

Edit: For what it's worth, I also tried stacking Enspell damage gear.

Sequence, Ayanmo+2, Ghostfyre Cape
No waist: 119
Orpheus (MD): 136

Levante, Ayanmo+2, Ghostfyre Cape
No waist: 134
Orpheus (MD): 154

Chatoyant, Ayanmo+2, Ghostfyre Cape
No waist: 136
Orpheus (MD): 156

So flat out affinity. Neet.

Orpheus's sash, sells to vendors for 19,250 gil :D