Dennis, if you examine the published sources which actually lab test receivers(e.g., Sound&Vision)you'll note that the noise levels found(more than 70dB down)are close enough to or actually at the theoretical limit as to be totally inaudible in normal use.
As Alan suggested, any advantage some separate preamps may have over some receivers involving a dB or two lower noise floor won't be audible. Engineers have learned how to design receivers combining excellent performance with moderate cost and they're in most cases the sensible cost-effective purchase.
