I would consider upgrading to XA30.8. The move to X250.5 will give you more power but it will most likely be a step backwards sonically.
Edit:
The XA25 is most likely enough for your Martens. Pass amps are conservatively rated to 0.01%THD. The XA25 has headroom to 75-80 watts per channel into 8ohm and around 130w into 4ohms. I think you should be fine.