I just got the FX Audio Tube-03.
First impressions, after the first hour.
Volume: If you turn the volume higher than 12 o’ clock, expect a ton of distortion.
Bass:
-If you don’t put the bass knob at 11:30 
or more, expect a total loss of sub-bass.
-It’s like the bass knob “starts from 0hz”. Turn it left all the way and it will erase everything from 0-200hz.
-If you turn it right a little bit, expect a pleasant bass boost. As if my T50RPs needed more bass… but here it is.
-More than half (12 o’ clock), and re-live the disgusting Honda Civic era of people with loud and shitty, distorting subwoofers in their trunk.
-But if you want, you can bass boost the crap out of everything and get a “old Beats by Dre simulator” by lowering the volume knob to 1/4 (nine o’clock? 
) and then turning the bass knob to the right. No distortion.
Treble:
-Maybe three clicks left again, 11:30-ish
-Well, that’s a surprise. Even with the knob all the way down, I feel like the Tube-03 adds a ton of “air” (10khz+) to everything (no “esses” sssibilance – higher than that!). Where the bass knob was just, basically “erasing” all the low frequencies, the treble knob just “pushes em down”… but this 10khz+ “air” stays. So whatever you play, it never sounds muffled.
Other infos:
-Not microphonic. I.e., if I pause the music and knock on the casing, or even touch the tubes, I don’t hear anything.
-I’m actually stress-testing it with powerful, sometimes “wall of sound” electronic music. So if I don’t hear distortion now, you, jazz amateur, certainly won’t hear atrocious distortion.
-My first impression was, actually “hey, this sounds clean”. And this 30$ thing also adds soundstage.
I was expecting “tubiness” to be smoothness, a higher amount of bass, and muffled sound. I got a bass boost knob, “air” and soundstage, and apparently clean sound. Definitely worth the 30$ for me. 
I’d just recommend this to everyone who has closed-backs or open-back inexpensive headphones with drivers “dying” at 10khz. Maybe this can help.