Surge protectors are mostly MOV based;
http://en.wikipedia.org/wiki/Varistor
Under normal conditions, they don't do any work, they sit there as if they didn't
exist waiting for a power spike to occur, then it shunts that spike. You can also
burn out MOV's when you exceed their rating but you'd never know they
went bad if they burned up unless they shorted and tripped a breaker/blew a fuse.
re: line conditioner
Just some caps and coils to form a filter on the AC input line. SQ from line conditioning is not guaranteed either, depends on
what problems exist on the power lines. If you have no AC line gremlins, then a line conditioner isn't doing
anything useful. Most if not all electronics have some form of filtering inside,
the degree of filtering is a variable. Some power supplies like SMPS has
these line filters by default, not necessarily to filter out incoming AC gremlins,
rather to keep the SMPS gremlins from coming out and contamination the AC
house voltage.
re: picture quality.
You have a better chance of seeing improvments in picture quality than sound quality so it
might be a good idea to try one.
