And - especially when you get into the better mics and preamps - the pairing of mic and preamp can be just as important if you are really trying to tweak the tracking. Preamp X may have a certain effect on the sound of Mic Y, but the effect it has on Mic Z is entirely different. Maybe it's rather sublime to talk about it on the econo-mixer level, but it's no less true; mics and preamps really need to be considered a coupled system just as much as monitors and room do on the other end. One can't really properly consider one without the other.
Again, maybe on smaller systems where there is only one kind of preamp available to the user (as in the many fine folks who have only mixer channel strips), that's only an academic discussion best left for the theorisis. But when one starts talking about buying mo betta preamps, they should take their mic locker (among other things) into consideration when they are doing their research.
Bringing that back home, I have to respectfully disagree a bit with a couple of the other responses; I believe you could easily hear a difference and improvement between your Yammie preamps and a dedicated pre that costs as little as $90/channel, and for $200/channel you'll be certain to be stepping up. They'll still be prosumer-grade pres with plenty of room for improvement compared to the real pro stuff, but even those prosumer grade pres will be a huge step up from what you are getting in a $25/channel mixer channel strip.
G.