Re: quality.. - some real answers.
Some real answers are.
Decent interconnects for analogue, well shielded. Mechanically reliable, do not allow signals to leak in. Avoid crosstalk.
Decent interconnects for digital, basically is it tough enough not to break, cause drop outs, fall out of sockets.
Decent stand for players with moving parts, make sure the turntable has a secure base. This can make a huge difference with SOME equipment.
Thick speaker cable of reasonable quality. again a case of making sure it doesn't colour the signal with high resistance.
Wall rugs is a good one, carpets is another.
Avoid signal cable adaptors, use a BNC plug rather than a phono BNC adaptor.
Uncompressed, lossless, or high bitrates.
All of these are a case of decent quality rather than expensive. Most important things are to not fall apart and to get the information from one place to another as well as possible. The most obvious places for different quality of cables are SCART leads, you can tell between a cheap and a well made. Any really poor interconnects can act as aerials.
Real jokes are gold plated optical cables, Monster, £40 HDMI cables (unless long). Bose.
Decent components do not have to be expensive, amps, about £100 a channel, speakers around £100 a box is a good starting point for a really good system.
Also do not be afraid of trying alternative sources, been cases of DVD players beating CD players at playing CDs.
For information my HDMI cables were given to me with TV but were £10 each. Analogue interconnects a couple of quid, (under £5 a set - but bought YEARS ago), speaker cable, can't remember how much I paid but nice and chunky. My most expensive cable is still a SCART lead.
Diminishing returns cuts in a lot lower than a lot of people think.