Well, I ended up making a project out of the hearing test.
First, the online tone generator was hard to work with. I used the music.ece site with a pair of Grado SR 60s plugged into the computer phones jack and heard bass down to 25hz and high frequencies up to 16.5khz and something beyond that, but am not sure I heard a change in frequency or just noise. Feeding the same source from the computer, into a B & O receiver and a pair of Def Tech Studio 350s, with a Def Tech SuperCube III sub, performance was similar, but only down to 35hz (there was some signal at 30hz).
I have several audio test discs and used a MoFi with 1/3 octave test tones played on a Sony PS3 into the B & O/Def Tech rig-results were the same as above with the Music.ece site (30hz to 16.5khz). Listening to the Grados through only the computer with this disc improved the response at 20 and 25hz, with no change in the high frequencies.
Finally, I played the MoFi disc on the big boy analog rig. 20hz was audible and 25hz was solid. 16.5hz sounded like on the B&O/computer rig and I heard nothing at 20khz. When I measured and set up this system, it rolled off above 16.5 khz, so the 20khz result is not surprising. I also can't hear sh*t beyond 16-17khz. The good news is my hearing is about the same, or only slightly worse compared to 10 years ago.
I gave up chasing the last 1/3 octave in high frequency performance years ago. I don't think a super tweeter will help me much, but would like to hear the effect, or at least try too.