I think the manufacturers specs for hum on your pre are -88db below full power. Whether or not this results in audible hum on most systems I can't answer, what I do know is that there is audible hum with either of two tube phono pre's that I've owned and with my current tubed pre. I've come to just except it as part of the cost of going the tubed route.
An option, assuming that you have plenty of gain in your system, would be to call manley and ask them about reducing the gain on your power amp, this is usually an easy thing to do but I'm not up on the manley's. If you have a friend who is handy with electrical stuff they could make a very inexpensive couple of voltage dividers for you to play with as a way of determining whether reducing system gain seems like an improvement. Tweaking down the gain on your power amp obvisously won't eliminate the hum but maybe take it to a point you are more ok with. Good luck.
An option, assuming that you have plenty of gain in your system, would be to call manley and ask them about reducing the gain on your power amp, this is usually an easy thing to do but I'm not up on the manley's. If you have a friend who is handy with electrical stuff they could make a very inexpensive couple of voltage dividers for you to play with as a way of determining whether reducing system gain seems like an improvement. Tweaking down the gain on your power amp obvisously won't eliminate the hum but maybe take it to a point you are more ok with. Good luck.