Hi Anthony,
There won’t be any issues with using the buffered inputs on the amp, but the quality won’t be as good as with the unbuffered input. As you are using the Lina DAC, you also don’t need the extra drive that comes from the buffered input on the amplifier. If you need some extra volume on the system, make sure the DAC Output Level is set to 6V not the 2V default, and that the amplifier is in high gain mode (the gain switch under the volume control set to the right hand side).
The buffered input is there for if you are using a DAC without sufficient drive to use the unbuffered inputs. The Lina system definitely has enough to drive the 1266 with the unbuffered input, so you should use this.