Harmonics don't stop at 20kHz and it's been shown that they can have a direct impact what we hear. Supertweeters do add the edge you are are wanting and would be well worth considering particularly if you are analog and the source extends higher than human hearing.
MillerCarbon might have been doing a hard sell for Townshend Maximum Supertweeters but a lot of what he said is pertinent.
I am enjoying my analog system, but what can I do to improve?
I currently have Technics 1200G turntable with Dynavector 17XD cartridge playing through Kitsune LCR 1 MK5 phono pre and Allnic L7000 preamp. My amps are Pass X350.5 and Benchmark AHB2 driving Sound Lab ESL speakers. My system sounds great, but I am wondering how I can take my system to another level. What do you think?