Being tight, I tried everything before I finally splashed out on Ultimate ear monitors. This was around 7 years ago, and they are without doubt the best solution. Plugged into my Sena which is connected to my Zumo, the sound is crystal clear at any speed, with the volume set at around the 30% mark.
At a 100% volume the sound actually hurts.
By the time I'd realised this, I had spent the same on rubbish that didn't work.
Good luck with your quest.................. You would not regret buying ultimate ear monitors once you had tried them.............. But it hurts to part with all the cash for something you hope will work, but are not convinced.