Loose fitting power cord.

I've used the electrical tape method as well. If it ever needs to be removed, it can be a sticky mess. Try some heat shrink. 

It will definitely affect sound quality if the power cord falls out.

Its not the outside of the plug but what's inside that's the problem. If the terminals inside the plug grip the pins on the component with enough tension the plug will not fall out no matter how sloppy the fit. Conversely, no matter how much electrical tape you wrap or how tight it stays in nothing like that will address the real problem of the electrical contacts being this weak. 

So you have a piece of crap power cord. Well there are a lot of them out there. Live and learn. The only real fix is replace the plug on the end of the cord. Or you could open it up and try bending the mating parts inside enough to get a better grip. 

What rubbish.  It's perfectly well known that many manufacturers stray slightly from the established standards of external size of plug and internal size of receptacle.  Others have reported the reverse problem, struggling to get a plug all the way in.  In the past I've had problems with Cardas PCs, also being a crap manufacturer, right?

Besides the electrical tape, which I've used successfully, you have also try to improvise something to support the cord near the plug.  The heavier the cord--garden hose variety--the more likely this is to happen.  I know the biggest model of Pangea causes problems.

Try plumbers Teflon tape. Easy to remove.