Mplayer has command-line equalizer, e. g. '-af equalizer=3:3:2:0:-3:-1:0:0:0:0' It is possible to add graphical settings for this?

It just so happens that gmusicbrowser stores the equalizer settings for the Gstreamer backend in a format that is exactly the same as Mplayer's. That makes it very easy to have the Gstreamer equalizer apply to Mplayer as well. Just insert this line in, at line 77:
push @cmd_and_args, '-af','equalizer='.$::Options{gst_equalizer} if $::Options{gst_use_equalizer};

Of course, this is only a workaround and has its limitations: you can't adjust the equalizer in realtime during playback, and in fact the equalizer controls will only be adjustable when you have the Gstreamer backend selected. But after you've adjusted the settings to your liking, you can just switch to Mplayer backend, keep the "Use equalizer" checkbox ticked and the equalizer will work.

Thanks. I just copied this settings from other player :)