The more gain a preamplifier is called on to produce, the harder it is to keep noise down. Even with modern electronic advancements, designing ultra-low noise preamps is no easy trick. Manufacturers need to invest a considerable effort in the R&D department, and ultra-low noise components don't come cheap either, so expect to pay more for higher quality preamplifiers. Pay particular attention to the preamplifier's noise characteristics - the lower the better!