You could use them as either front L/R or surround L/R.
The problem using them as fronts is that you would want a center speaker voiced to work with them and there may never have been one. Try to find a KLH center with the same tweeter as your speakers if you can.
If you use them as rears it's probably overkill and may be hard to place them. You don't need large speakers as surrounds and while it's good if the match it's not that important.
They should not be difficult to drive so pretty much any av receiver will work with them.
You can get a subwoofer but it you use the KLH as fronts you would need a large one. You could add that later.