Stacking a device with a low level phono stage in it near a power amp other powered components or devices is the main problem I've encountered, but certainly many other similar kinds of problems are possible when multiple electronic devices are in close proximity to each other.
Having said that, I have my components stacked pretty tightly and there is no trace of noise or distortion as a result in my system currently.
I did have to move things around a bit recently and add some external magnetic field shielding to my step up device for my low output MC phono cartridge to eliminate very noticeable hum otherwise (see postings on my system for details).
In general I would recommend avoiding stacking components in close proximity if it can be avoided.
Having said that, I have my components stacked pretty tightly and there is no trace of noise or distortion as a result in my system currently.
I did have to move things around a bit recently and add some external magnetic field shielding to my step up device for my low output MC phono cartridge to eliminate very noticeable hum otherwise (see postings on my system for details).
In general I would recommend avoiding stacking components in close proximity if it can be avoided.