Ever since the early 90s, I've been using button battery cassette based demagnetizers to maintain my cassette decks along with cleaning my heads with alcohol and a Q-Tip.
I had a tape really mess up my deck's heads. Cleaned them and sounded extremely muffled. Demagnetized with the cassette and sounded much better. However every time I need to demagnetize, the cassettes never sounded that great, especially next to digital or vinyl.
Well I was about to give up on cassettes all together but I have about 60 prerecorded and 20 Metal tapes I recorded myself.
So before selling everything to a local a vintage used equipment, record, tape and CD shop, I tried one last ditch effect. That being a wand based demagnetizer.
WOW, what a difference. My cassettes came back to life. I hear cymbals and all kinds of treble now. I honestly thought my cassettes were worn out being ~30 years old as well as my JVC TD-W318 dual cassette deck. No way, they sound great. Not as good as vinyl or digital but very good. That stupid button battery cassette demagnetizer wasn't fully demagnetizing the heads and leaving some residual. I have been thinking for decades that my deck and tapes were just worn out but it was the demagnetizer. I also had some dirty switches in the deck that kept it turning off during playback. Got the switches working good just by working them a lot which likely cleaned the oxidation off. I've missed out for decades.
I will say that my Metal Type IV tapes sound bad. They didn't last as the Type 1 normal tapes did. I thought Metal tapes were suppose to last longer but after some investigating I guess there is a chemical reaction with them that deteriorates the sound quality after about 20 years.