There is normally no detectable heating of an audio transformer used at normal audio line levels, which describes the level coming from your output.
The only way to distort the sound through the transformer is to over-drive it, for example if you hooked up the output of a 10-Watt or higher power amplifier and tried to run more wattage than the transformer was designed to handle, it would over-heat, distort, and probably fail. But you are not doing anything like that.
If badly mis-matched an audio transformer can distort, but your output is exactly within the design range for which this transformer was intended, so it should sound the same as if no transformer was used, except you'll have the advantages of the long line being balanced. I think you'll like it.
sorry by "hot" i meant the signal be too high a voltage for the amt-3000 input and causing clipping.
however this will not be the case because transformer is actually going to be used to go from balanced -> unbalanced so its gonna attenuate the signal. in my bench tests i was using it to go unbalanced -> balanced ( i dont have a balanced source to test with) and this is when it has gain.
here is the assembled box: 
i went with a TRS 1/4" jack rather then xlr because i wanted to use as small an enclosure as possible. i really hope there will be room for this thing in the weatherproof box on my roof. right now it has the transmitter, the ac adapter, the power cable and the audio cable all squeezed in there and its almost full. would it be okay to have the power adapter in my studio and then run a long coaxial cable up to the roof rather than running a 110VAC power line up to the transmitter?
I have heard of others running long voltage lines so the power supply was able to be far away from the transmitter enclosure. I'm pretty sure it's a good idea.
cool. i guess i was worried about 1) the power line acting as an antenna and picking up unwanted signals and 2) a long cable after the power transformer having resistance and resulting in a lower voltage to the transmitter.
do you think either of those are real concerns?
To keep RF out of the power line you need those inexpensive little RF chokes from Radio Shack that just clip or clamp right on your power wire. There are several types and sizes of them, and people also use them in computer wiring, video wiring and telephone wiring to eliminate interference.
iirc DC line drop is 2v per 100ft on 14ga stranded copper easy enough to verify with a lab supply your intended power cable and a DMM
"i was worried about 1) the power line acting as an antenna and picking up unwanted signals and 2) a long cable after the power transformer having resistance and resulting in a lower voltage to the transmitter."
You can put a line choke on the power cable feeding the box upstairs and that will kill any RF interference on the line. Put the choke on the line at the point of entry inside the box.
Unless your power line is ridiculously long, I would not worry at all about any voltage loss with the 120vac feeding the box. Just make sure everything is weather proof and water tight.
Do frequent inspections of the box for any signs of leakage into the box or worn/broken feed through seals.
RFB
i installed the XLR cable and transformer last night. the audio quality is noticeably improved but i am still getting interference from the mystery spanish station.
i guess i need to install a choke on the power line. can someone post a picture of a power line rf choke so i know what to look for?
The Radio Shack Snap-Together Ferrite Choke Core is Pt.# 273-105
http://www.radioshack.com/product/index.jsp?productId=2103222
cool. would any of these be suitable?
http://www.allelectronics.com/make-a-store/category/235/Ferrites/1.html
Yes.
Those are the same thing.