Usage of prevalent and pervasive

I can't distinguish the usage of these two words: pervasive and prevalent. Almost all dictionaries define these words' meaning as spreading throughout [a medium].

One forum I have come across states: (https://www.englishforums.com/English/PrevalentVsPervasive/bdnxdv/post.htm)

Prevalent has to do with expanse of effect and pervasive has to do with depth of effect.

Another forum claims: (http://ask.fm/The_YUNiversity/answer/120917623170)

Prevalent carries the additional meaning of being superior to something else (as shown by the associated verb prevail).

Pervasive generally has a negative connotation and is often used to describe unwelcome things.

These two explanations do not concur at all, though that need not make either wrong. But could someone please indicate the true facts of the matter?

I think the difference—and there is a difference—may be put down to the fact that one tends to use pervasive when making value judgments.

The adjective prevalent is neutral with regard to judgmentality. A thing is prevalent or it is not, without ascribing any affect* to the thing itself. No power is at work ascribable to an agent other than one that is accidental or indifferent.

Volleyball is prevalent on California beaches.

The person uttering that statement makes no value judgment about volleyball, and may be indifferent to it, hostile to it, or in support of it—but we'll never know that until the person elaborates. And the statement could simply end there, an item in a list of the characteristics of California beaches.

Volleyball is pervasive on California beaches.

Now we sense that someone has an axe to grind regarding beach volleyball, that an attitude is about to be revealed, whether pro or con (most likely con, but not necessarily so). The word pervasive in effect functions as a setup for something to follow. The issue will not be settled if the speaker does not go on to elaborate.

Note that this is a subtle distinction, but it is one that a careful listener or reader should be on the alert for. Prevalent could easily be used to set up an evaluation, and pervasive could be used just as innocently. But in all my reading, what I describe above is what I have seen most.

*_{If you think I mean effect here, see the noun listing for affect.}