2 things.
You will not enjoy what a great preamp does with vocals without a great vocalist. You need to bring it to the table to hear what a great preamp will do.
You will not enjoy what a great preamp does with vocals if you do not have a darn good set of monitors or a darn good home stereo.
I think that is why most people can easily convince them selves that a industrial grade preamp can compare favorably to a no holds barred, best of breed preamp.
If you're happy... be happy.
If you have a great vocalist to work with than it's worth listening to the details and enjoying yourself.
I think the $500 price point is too low to bother with if you already have some nice industrial grade stuff. The industrial stuff will compare favorably to that stuff.
Here's a unit I'd recommend without hesitation:
http://www.greweb.com/product.cfm?ID=20&type=0
It's nothing very special... but it's also the real deal.
If you can't hear the difference between this and the stuff spoken of as almost as good... then you don't need it.
If you can... and you get used to it... you are going to want it.
Witnessing people's reactions to the recent Sound on Sound test has been a revelation of sorts. The folks that use that test and article to bolster their confidence don't bother to question the fact that
1) There wasn't an actual great preamp used in the comparison. (I own a API 3124+... sounds great on drums ;-), I've worked on real 1073's... the AMS clone is expensive... but there's better sounding 1073 clones)
2) The test wasn't "blind", and it was not a "test".
3) The sound source sounded so bad and lacking in dynamics that it masked any nuance you might hope to appreciate. A player piano to test preamps? Really? It's a percussion instrument being played by a robot.
It reinforced my impression that people that don't know, don't really want to know.
Frank, I'd say that the primary reason to spend the money will be if you have a singer that can really work the mic, work the preamp, and make it matter. You'll know when it's time because you'll start realizing that the *mackie* grade stuff all sounds the same and you'll want something better.
Your question, as I interpret it, is a great one;
"How much do you have to spend to really get past industrial grade sound?". The answer may not be comforting, but there is an answer.
Good luck in your endeavors.
best regards,
mike