Tubes v. SS: can you mix and match?

Gentlepeoples - I was under the impression that a tube pre was always mated to a tube amp. Likewise, a SS pre would be mated to a SS amp. Of course, different makes and models of pres and amps could be mated, but I thought tubes should stick with tubes and SS should stick with SS, and never the two should meet. But this doesn't always appear to be the case with some of you. So, when and how are tubes mated with SS? Is it just a question of what sounds good? Are there any parameters at all?