i hear this over and over again (that using a sub exagerates the bass and wrecks your mix) and it is complete bunk.

with the crossover properly set, a sub extends the frequency response of your monitoring chain down to the bottom 2 octaves that many small monitors can't reach. if your stuff is going to be played at the club on 18" woofers, you definitely need to know what's happening in the lower frequencies, or you will get a big surprise.

set improperly, a sub will double up on the upper bass (with both the monitors and sub pushing the same frequencies), which will cause problems, but setting it up properly isn't really that hard. you can use a frequency analyzer and a microphone, or just use your ear. play a series of notes from a keyboard, and as they go down, the sound should transition from the monitors to the sub smoothly, with no bump or gap in response. if you notice some notes are louder or quieter than others, adjust the crossover down or up until every note sounds at the same volume.