Setting Up and Testing New Monitors

Cindy, those speakers are going to be just fine, so don't let anybody second-guess you on your choice. We all have to work with what we can afford, and plenty of commercial records have been mixed on KRKs.
 
The speakers themselves aren't the problem. It's your room. 
 
Consider this: even entry-level monitors like yours are flat across the most critical frequency range, perhaps varying by 3 dB or less. Not a problem. But your room may be causing frequency dips and peaks of 30 dB or more! That's a problem.
 
The good news is you don't need an oscilloscope or spectrum analyzer to identify problems. A good microphone is all you need. Here's an old post I made on how to do some simple testing using only SONAR and a free spectrum analyzer plugin.
 
Next step up in sophistication is to use something like the free Room EQ Wizard (REW). This kind of tool is needed because it shows frequency response over time. If certain frequencies ring out longer than others, it will distort what you hear and lead you to make incorrect adjustments that might make things sound better on your speakers but no one else's.
 
But the very best thing you can do costs nothing at all and is pleasurable to boot: spend as much time as you can listening to your favorite recordings through those speakers while sitting at the mix position. That's it. And it truly works, because what you're doing is subconsciously training your ears and brain to recognize what a good mix sounds like on your system and in your room. This is explained well in a book called "Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms" by Floyd Toole, the now-retired head of research for Harmon.
 
He also explains how to position a subwoofer, and why where you place it is more important than the speaker itself. Low frequencies are especially troublesome. It takes a great deal of acoustic treatment to tame them, and most people don't have the space or the budget to do it right. Placing your sub will be the most critical choice you make. Knowing where your room's worst resonances are will be the most important piece of information you can arm yourself with.