It was not an online mastering thing, but a human in Europe.

I hope the advise we have given you, like "ya sounds a little hissy"...or "too muddy man". has helped .
Still haven't given us any idea about how you mastered it. IE- the FX used, sample rates, RMS on Mixdown.
these are things that are critical to know if you want help.

once you take all your tracks and mix them down to a 2 track 24 bit stereo file (leaving enough headroom for master/Look up Peak/RMS Mixdown levels) Import that file back in to sonar (I typically use a new project/cuts down on resources) Then it all depends on what you have for plug ins. if you are using all sonar plug ins then you'll have to round up the best ones you have. If you have other plug ins like Waves, exc then your options are a lot better. My typical mastering chain looks like this:
 
My Mastering Chain consists of:
1-The SSL Bus compressor first.
1.5- then sometimes followed by the CA-2A for Analog coloring= a 1 to 1.5 db push / very subtle.
2- Linear EQ to drop off/filter APPROX 30HZ and under.
3- Then the L3.
4- Followed by the L2.
5- Some  more light Analog coloring.
6-The Secret sauce.
 
I also (personal opinion) Insert my mastering chain in the Tracks FX.
This way I can use the Master bus for Metering tools. IMO, it just makes a cleaner area to work in.
 
your mix sounded like it may have been missing Linear EQ. you need some sort of high pass filter to clean the low end. this will allow more boost with less distortion, dynamic loss, burn out. It also sounded like the Mixdown was too loud (RMS).

So many good notes to be sure.. 
 
For me, mastering has become the thing I am taking the most time to learn, and it comes in baby steps - so consider me a beginner. What I might share are the two things that helped me the most so far.  The answer to your question, from my perspective is to  choose a reference piece of recorded music and have that up to A-B with.  It takes some of the environment and hardware out of the equation. I would add to make sure you listen at 85db to your monitors for pop, rock, country and the like. If you have subs, make sure they also are set for 85db. My test tools aren't perfect - a set of pink and white noise wave files and a freeware download of NIOSH SLM for my iPhone. 
 
Give the two ideas a shot - it works.  In my case while  I had a technical fundamental knowledge of physics and electronics - and thought I knew each tools effect, I really didn't know enough to HEAR and translate it into the actual answer or at least, a close proximity. That was humbling, but valuable - and I'm still learning and am '20 to 30 percent better'    :o)

I hope you didn't pay too much for the "pro" job.  IMHO, you did a better job yourself.
John B.