A "mic input" is a "mic preamp". The preamp amplifies the low level microphone output to line level. Very simple sounding, but just as critical to your sound as the microphone itself. If you're not happy with your sound, then new a new microphone and/or new mic preamp is probably in order (at some point anyhow). The mic pres in the radioshack mixer probably suck, but who knows, ever now and then radioshack gets something right out of sheer luck (or good outsourcing).
Slackmaster 2000