Re transformer (etc) hum, this is a facinating - not to mention
frustrating! subject, and one with various causes. And usually it's not necessarily the transformer itself as such the main culprit...including Hypsos's. So often it will be down to electronic/electrical equipment within one's environment and which can sometimes be very hard to trace...not to mention crud in the mains supply.
For example, the gigantic trafo in my Balanced Mains power conditioner suddenly went from whisper quiet to loud hum for no apparent reason. After much searching and head scratching the culprit turned out to be a pond air pump located outside in my greenhouse! Mind you, there was no hint of disturbance in the sound system...probably courtesy of the conditioner's 'Advanced Filter System'.
And so, if unable to be thus fortunate in locating the offending gremlin and mechanical hum is still present, I suggest using what I personally use on
all electronic hifi equipment anyway - ie adhesive 'Dynamat Xtreme' sound deadening mat, or the cheaper alternative 'Silent Coat' wherever possible inside or outside the units. This amazing stuff will kill most noise from component vibration/resonances...with beneficial effect on
sound quality as well as annoying mechanical intrusion. In addition, applying this mat to equipment feet will be more effective than any uber expensive isolation feet IMHO, and far cheaper lol...even if also adding some hard wood (like oak) cone ones. Plus, of course, standing units on solid hardwood platforms...something like 2cm solid oak being just as good IMO as (again)
uber expensive 'fancy' isolation ones. Using these methods (along with well shielded/dual construction cables) has enabled me to do the unthinkable re stacking units - ie Chord's TT2
atop mscaler(!!), let alone Hypsos atop OOR...and without the slightest hint of hum or any other noise related degradation to sound performance.
In conclusion, and as mentioned at the beginning, I believe one should look into these areas first before worrying too much if a particular unit is of itself
faulty...the problem most likely lives somewhere else...