This is why guitar amplifiers are often full range or have tweeters. This makes them sound better. The classic guitar amplifiers did not cut frequency response at some mythical level. Humans can perceive sound above 20 kHz, and many musical instruments emit sound up to 40 kHz.
I have never seen a guitar amp with tweeters - and I've seen a lot of guitar amps. When I've encountered guitar amps being run into full range PA cabs (with tweeters) they've always sounded awful.I don't know where you have an idea guitar amp use tweeter. You even know what you are talking or just arguing?
JBL's guitar speakers are labelled as "extended range" but I couldn't find any reliable data as to what the range is. I doubt it goes to anything like 20kHz. All the classic Celestion guitar speakers go up to about 5kHz.
https://celestion.com/product/1/vintage_30/
Ad hominem attacks are not ideal but some statements are so so bombastic and plain wrong as to invite them.Why not respond with science and engineering, not engage in ad hominem attacks.
This spec sheet from Electro Voice states the frequency response as "80-5000Hz".Some of the odd order harmonics produced by instruments are unpleasant, but the even order ones are nice. Classic full-range drivers (Electro-Voice, JBL, University, etc.) are often used in guitar amplifiers for this reason. The amplifiers are not necessarily cut of at 5 kHz.
http://pdfasset.owneriq.net/f/75/f7.../f75c2e60-a0c9-4b36-ad81-625bb0af689c-bg1.png