I like to use http://www.audiocheck.net/audiotests_frequencycheckhigh.php when I'm playing around and do the blind test to help verify.
Obviously calibration is going to be a big factor. Since you are setting the volume yourself, your results may or may not be so accurate.
These types of tests aside, even if you could hear up to 20khz in a test with no other frequencies present, would you actually be able to notice in the average song? I think I would have a hard time. Maybe others wouldn't.
I'm considering picking this up to go along side the Magni. Hopefully it offers a different perspective on my music and isn't simply a redundant piece. From the current impressions, I think it'll be a nice add on piece for the price. Like one of the other members had mentioned, I was also considering a project ember to bring the tube factor into my music experience. Not having to deal with tube rolling looks like it could be beneficial but possibly restricting. The price and ease of use is winning the fight.