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It's fine - I have x200 signatures and they don't get hot so I presume same for x300 and I've never heard anyone say they get hot. The fans come on to cool them if the volume is really high and there are two fans per unit. When they turn on the fans rev up. But I've never heard the fans come on in listening, probably because I don't push my speakers hard. The only component that does get very got and should not be stacked under something is the cyrus dac xp signature. It runs at 20watts in standby and 22 watts when on, so that's warm!
my only reservation with stacking cyrus units is that you won't be able to dust between them. I say this because the bushed black and quartz silver finishes on the top are a rough painted finish, such that dust does 'key' into the surface. With years of use the dust can 'burn' onto the surface leading to discolouration, from ones I've seen people sell. I can only put this down to not cleaning the dust away. So my advice would be put them in a decent rack spaced apart for this reason alone if you really like to look after hi fi. There are also other reasons to space and isolate for acoustic damping, as its known that physical vibration can affect dacs and electrical vibration too.
if you want a really good rack I'd recommend the tri arbour range to suit cyrus - check out simon little furniture. Alternatively hi fi racks do them for Cyrus units but cooling not as good as the tri arbour racks. You can get tri arbour made with additional shelves too.