At first glance I would say that the Bluetooth implementation in the speakers doesn't support a 48kHz sample rate. The specs of the MR1 speakers say:
"BLUETOOTH
Built in high quality Bluetooth Audio receiver with aptX provides CD quality sound with compatible devices"
CD Quality is 16bit 44.1kHz, so maybe that is all it can support. That would be unusual for Bluetooth, particularly using the aptX Codec, which normally does support 48kHz. In fact usually, if there is a problem with Sample Rate, Bluetooth devices tend to support 48kHz but not 44.1kHz. If the MR1 doesn't support 48 kHz, then it doesn't support the aptX standard fully. That would be strange as they put the Qualcomm aptX logo in their User Guide. See
https://www.aptx.com/aptxThe fact that they don't clarify anywhere on their website what the speakers actually can support via Bluetooth, or even wired, would make me search for more information. The website is mostly marketing fluff aimed at people who worry more about the look of something than the performance. Even the User Guide provides no more information.
I think you need to contact Ruark and ask what the speakers support, both for wired and Bluetooth connections.
In the meantime have MC transcode everything to 44.1 kHz when connected to the MR1 speakers.