Standing waves

Well, on the one hand, standing waves cause the largest measurable deviation from an ideal response in sound quality. On the other hand, problems get worse as you go down in frequency so as a result smaller speakers, with higher cut-offs tend to sound better.

If you want your listening room to be close to flat from 16 Hz to 30kHz they must be dealt with.

But this is all general theory. Better question is what is your specific problem and how to deal with it. :)

The first question needing to be answered is the dimensions of the room. Generally, the smaller the room, the worse the standing waves (eigenmodes).

They're about as detrimental as your ears think that they are...really.