add preamp to EQ

backend/app.go

@@ -186,6 +186,7 @@ func (a *App) setupMPV() error {
 	a.Player.SetAudioExclusive(a.Config.LocalPlayback.AudioExclusive)
 
 	eq := &player.ISO15BandEqualizer{
+		EQPreamp: a.Config.LocalPlayback.EqualizerPreamp,
 		Disabled: !a.Config.LocalPlayback.EqualizerEnabled,
 	}
 	copy(eq.BandGains[:], a.Config.LocalPlayback.GraphicEqualizerBands)

backend/config.go

@@ -75,6 +75,7 @@ type LocalPlaybackConfig struct {
 	InMemoryCacheSizeMB   int
 	Volume                int
 	EqualizerEnabled      bool
+	EqualizerPreamp       float64
 	GraphicEqualizerBands []float64
 }
 
@@ -158,6 +159,7 @@ func DefaultConfig(appVersionTag string) *Config {
 			InMemoryCacheSizeMB:   30,
 			Volume:                100,
 			EqualizerEnabled:      false,
+			EqualizerPreamp:       0,
 			GraphicEqualizerBands: make([]float64, 15),
 		},
 		Scrobbling: ScrobbleConfig{