I recently built a pair of Lyra speakers.(They sound GREAT!) I noticed that one of the drivers could handle 50 watts RMS. Might this be a problem with an amp that is capable of 70 watts RMS?

it's not likely a problem because by the time you provide a full 70watts to your speakers they probably would be distorting so bad you would have turned the volume down anyway. If your still concerned , you could always fuse the speakers. I have a luxman amp that provides way more power to my speakers than they are rated for and I listen to them at very high volumes - no problems yet.