Hi My bass is a Boosey Hawkes Ply. KnK pickup + clickie. I play in a Roots rock band and a RnR/Billy band both with loud drummers, mostly on small stages/bars. (Standing close to my rig, GK and 4x10 31 band graphic) I have tried most of the above methods and they all have an effect on feedback. The F hole covers (Shaped out of a gardeners kneeling mat) worked best……and then I stumbled on a post in the Upright Bass Players Union Facebook page, this was all about double sound posts. this caused a bit of a stir,with some people for, some against. So I thought I would give it a go. I got some spruce dowel from BandQ cut it to length and fitted it between the bass bar and the Bass F hole. Bingo! no feedback at all. I can stand directly in front of the cab and still own the drummer!! the 31 band EQ helps but is set mostly flat ( I only occasionally have to adjust the sliders) The electric tone of the bass hasn't changed. The trade off is loosing the acoustic tone of the bass but after a while it gets easy to fit/remove the post. It costs next to nothing to try, it may not suit you or your bass but it works for me!