So i figured I would post an update.
I had read that using an amp with a 2 prong cord would probably eliminate the ground loop issue. It just happened that an Emotiva UPA-200 was available on craigslist for a good price, and when I looked at the pictures, the power connection on the back of the amp was only a 2 prong connection. So I picked it up and gave it a try. The cord that came with the Emotiva had a 3 prong plug, so it was a 2 prong connection on one end and a 3 prong on the other. However, since there was no ground connection at the amp, it is likely working as a 2 prong connection would.
The ground loop hum was much quieter with the Emotiva than it was with the Audiosource, but it was still present. I could only hear it within a couple feet of the speaker where previously i could hear it throughout the room. So I decided to hook up the ground loop isolator that had helped so much with the Audiosource, and it actually magnified the ground loop noise with the Emotiva a lot. It was louder than the original hum with the Audiosource without the isolator! Go figure......
However, I did find the root cause of the ground loop. I unplugged the cable wire from the digital converter in the same room and bingo! I have no idea why I didnt try that before, after all i had read about cable wires being the cause. I guess since I had previously isolated the TV and cable from the audio system, I didnt think it could be the cause, but it definitely was.
Since I found out what the cause of the loop was, it comes down to which amp I like the sound of better. So I did a quick comparison and it sounded like there was a distinct difference between the 2. I didn't do any level matching, etc. so it really wasnt a scientific test. I played a few songs that I know really well and use a lot for demo'ing speakers (dave matthews and zac brown). I found the emotiva to be more detailed and a bit more forward sounding, where the audiosource amp 3 was a bit more laid back and warmer sounding. The Audiosource seemed to be missing something in the upper midrange which left my ears wanting something more, almost like a hole in that range. This was strange to me because I typically like a more laid back, warm sound, but in this case I felt like it was missing something in comparison to the Emotiva. Perhaps it was the placebo effect, or the lack of level matching that made the difference, but if you read about Audiosource, the general consensus is that they are more laid back and warmer sounding.
Anyway......they are both great sounding amps (powerful and clean), and the difference in sound seemed noticeable, but not night and day or anything. The Emotiva is 125 w/ch where the Audiosource is 150 w/ch. The advantages to the Audiosource are that it has A/B switching and its spec'd to run down to 2 ohms, where the Emotiva only has one set of speaker outputs and is spec'd down to 4 ohm. I guess I will hold on to both for a while and listen to them a bit more before I make a decision.
Unfortunately I will have to sell one of them, because that was the deal with the girlfriend when I bought the Emotiva.
Thanks again to the guys with original advice regarding the pre-amp, and Arny for all your help with the ground loop issue.