Speakers do not come with volume buttons. Therefore a single speaker will accept anything you feed it.
However, a multiple speaker set (bass, center, tweeter, etc) will have some sort of electronic schematic called a filter to cancel out all frequencies it was not designed to handle). This can be applied either per speaker (which is the best) or on a single schematic print inside the speaker box (which is the cheapest). There are of course really crappy filters and there are superiour filters. Depends on price and schematic.
What is more important is that the impedance (resistance) as already was mentioned. You need the same impedance at your output as on your input.
There are a lot of professionals and semi-professionals that can give you very good advice on what to connect and what to buy, but perhaps it is more wise to follow a golden rule:
Do you think the sound is good enough for you or not?
If so, do not upgrade your system at all.