They do this because they are mainly designed to push small earbuds that have poor sound quality and need the boost to get decent sound out of them. This is a bad thing when hooking up to a bigger speaker that does not need any boost. … Yamaha receivers, Marantz, Harman Kardon to Boston Acoustic speakers, Advent, Infinity just to name a few …
https://www.amazon.com/Marshall-Kilburn-Portable-Bluetooth-Speaker/dp/B00ZY1J1JG