Dropouts when using 64Gb SDXC

I’m using Volumio with

  • Raspberry Pi B+ and HifiBerry Digi B+
  • Raspberry Pi 2+ and IQAudIO DAC+

Initially, I created a 16GB SDHC card and copied some music to the /mnt/NAS/ directory. Playback was perfect on both devices.
Then I copied the image to a 64GB SDXC card (Lexar - class 10) and used the instructions (in the FAQ) to expand the partition.

When I use that 64GB card in either RPi, I get dropouts playing the installed music. However, playing the same music from an attached USB drive has no problems.

I have tried increasing the buffer to 4MB but it made no difference. I suspect that the read-speed of the card is not sufficient. How can I test this theory?

Would there be any benefit in creating and mounting another partition on the SD card?

Any other suggestions?