Your MC1502 should have plenty of power for either Fyne. Depending on what type of music you typically listen to, the damping factor of 18 may struggle to control the larger 15” driver. Sparse instrumentation and less dense material (I.e. jazz, typical “audiophile expo” recordings with one instrument and a vocal track) would probably sound glorious. Denser material at higher SPL (metal, grunge) may result in muddy, ill defined bass.
I “downgraded” my tube amp (PL Evo 300) when moving to the SS Kinki Studio EX-M7, and it resulted in a noticeable sound upgrade for my Arden’s. More definition, better PRaT.
As a Mc owner and fan, I’m not tying to steer you off the 1502, but you could sell that for $9000, pick up something like the EX-M7 for $3000, which would cover the cost difference of the Fynes, with a couple grand left in your pocket.