That's funny, because I consider the dubs for series like Cowboy Bebop, Outlaw Star, Black Lagoon, and Yu Yu Hakusho (just to name a few), to be much more preferable to listen to than their Japanese counterparts (and I saw the subs for all of those anime, except for YYH, before seeing the dubs for them).
What you said is only a matter of opinion. The sub may be the original, but that doesn't mean that the dub will never sound better to anyone, since there are lots of dubs which some people like better than the subs for certain anime, in their own opinions. It all comes down to one's own opinion, and when it comes to dubs vs. subs, there are rarely any things that you can say are facts in terms of comparing their quality to one another, for most anime that have dubs to compare to the original version.
But, I'm getting off-topic here, so to get back on-topic:
Honestly, I cannot say, and it would be hard for anyone else too, honestly, according to a question like this. The thing is, its asking if the dub actors themselves are better, worse, or equal to pre-lay voice actors, in terms of quality. As far as the quality of the voice-acting goes, pre-lay is almost always better than dubs, but that right there is obvious, since its an unfair comparison. With pre-lay, voice actors don't have to worry about matching any pre-animated lip-flaps, whereas in dubs, voice actors basically do need to do that, and that in itself makes the voice acting process much more tricky when it comes to dubbing an anime, or any foreign cartoon, for that matter.
Just that minor annoyance of having to match lip-flap movement alone can put up a significant limit on what a voice actor can fit into their lines, and what kind of tones they can use, in order to be able to match the lip-flap animations. In this way, the quality of the voice acting can be affected, and even if its still good, it wouldn't be quite as good as if the voice actor could be much more freely expressive, as they could with pre-lay voice acting. And this brings me to my point. If you compare the voices done by a good voice actor who only does dubs to that of a good voice actor who only does pre-lay roles, and see that the pre-lay roles sound significantly better, you still cannot measure that dub voice actor's full worth as a voice actor, until you hear how that voice actor does in pre-lay roles himself, and compare it to the other VA. And the same goes for the other VA, in that you would have to compare his attempt and dub voice acting to that of the dub-only VA, to get just a single accurate comparison between their quality as voice actors.
So, since a lot of dub voice actors usually only stick to dubs, and a lot of pre-lay voice actors usually only stick mostly to pre-lay work (obviously), you can't compare their actual quality as voice actors between dub and pre-lay voice actors in particular.
Having said that, though, I'd say that it depenRAB on which particular voice actor you're comparing to another. For example, if you take someone like Steven J. Blum, who has done both dubs and pre-lay voice acting, and compare him to someone like Kevin Conroy, who basically did mostly pre-lay work, but did try his hand at something dub-like by having to match the lip-flaps of the Bruce/Batman in the Batman: Gotham Knight anime, then you could make an accurate comparison in their overall quality of voice acting in portraying their respective characters, and say that they are of equal quality as voice actors (at least IMO).
Unfortunately, though, you cannot make these comparisons between most dub and pre-lay voice actors.
Well, that's all only just MY OPINION, of course.