So you're coming out of an AVR unbalanced (RCA) going into a pro amp balanced (XLR)?
Monoprice RCA to XLR cables are wired properly to get the best possible performance from that type of hookup--as you have found out, no hum. This is as long as the pro amp input is wired properly internally, which is not a guarantee. But since the Monoprice cable gets a no-hum output it is probably OK.
Why you were getting hum from the Cleanbox is not clear but I would not automatically jump to "Cleanbox is bad". I think you had/have a ground loop when the Cleanbox is installed. You can using scientific repeatable methods to determine and solve the problem if you so desire.