Thanks @BaffleChallenged - I did much the same, although I incremented the script 1hz at a time for 20000 readings across the spectrum:
for(var idx = 20; idx <= 20000; idx++) // 20hz to 20khz, 1hz increments
The script takes about ~10 minutes to complete. Here was the output from a little LM1875 chip kit driving an 8 ohm dummy load. Neat! This was before I added the conversion from dB to % for THD (thanks this website for the explanation!), but it worked out to an average of -62dB, or 0.079% THD. Not too shabby for an eBay kit running in the open air.
Another useful addition I added was to average all the THD values to give you a single value to compare against, as well as formatting the output into something more CSV friendly in case the results need to be saved and plotted elsewhere. I'll upload my script changes as well when I'm back at my laptop.