The first piece of advice I will offer is, don't add a sub to your setup. Ill come back to this point in a bit.
One of the most important tasks when first starting out mixing is to get to know you system. This is partially done when you pick monitors but in reality it does not matter what monitors you have as long as you know what they will produce. I have a few sample songs that I use to test out new systems. They are songs that provide a basis of reference for me, songs that I know so well I can easily use them to compare speakers my favorite is Steely Dan's "Hey Nineteen". A collection of test songs should cross all forms of music and be different in their sounds. Why you pick a test song is your call and it does not matter to anyone but you what you pick. Now go listen to that song in lots of places, then finally listen to it in your mix room to build a good basis for your mixes.
If you monitors are flat on the bass and you know that, you will have to do some predictive mixing but this can be dangerous. I will offer a few different solutions that may mitigate/solve your problems unfortunately some may be cost prohibitive.
Buy a second (or second and third) set of monitors: Unfortunately this is the expensive option but you will find this in many studios and is the best way to check your mixes. In this case I would chose 2 somewhat different monitors so that you can compare your mixes on the spot.
Mix On The Distributable Media: This is a bit of a lost art but back in the day engineers used to cary around Mono Boxes (basically the speaker part of an AM radio) to mix through since thats where the audio ended up eventually. Keeping in mind that you are mixing for the masses some times its not bad to demo your mixes through iPod headphones (or computer speakers) when mixing down. A mix that sounds great on $10,000 speakers is worth nothing if it sounds bad on iPod headphones since thats how most people will consume it.
Leave your monitors alone: EQ'ing your monitors wont fix any issues (well it may mitigate them a bit) but again we come back to the fact that you need to get to know your setup and how it responds in the real world. If you EQ your monitors you will just color them a bit and create a new slate to mix from a new slate that you will have to learn and become accustomed to.
Why I advocate for not adding a sub: as mentioned earlier most people listen to music through headphones or basic stereo setups. Adding a sub may cause you to mix you bass to light since you have added effectively a bass booster system.