There is a known issue that occurs when using the Avid Mbox Pro with Focal CMS 50 monitors: An audible noise floor exists at any monitor volume level. This issue is only known to occur between the Mbox Pro and the CMS 50 monitors. No other monitor has this issue with the Mbox Pro and no other interface has this problem with the CMS 50s.
If you happen to have these two products, you can reduce the loudness of the noise floor by lowering the input sensitivity on the Focals and increasing the Mbox Pro output level to achieve desired loudness. However, this will not make the problem go away.