Perhaps if sliders can't be implemented in the GUI, maybe have the default pans in the emulator set to something like this? Or maybe have it shown as an option in the .INI file?
I have tested these settings across the various sound types (I went on emor3j's wonderful chiptune archive site and nabbed the .NFS files for Akumajou Densetsu to test and tweak my settings for VRC6, Lagrange Point for VRC7, Just Breed for MMC5, Gimmick! for FME7, D.D.S. Megami Tensei II for N106 and the Famicom Disk System version of Zelda no Densetsu to test and tweak the extra FDS sound channels. I believe these are quite satisfactory pan and volume settings. Sorry for the sheer length of text, but since I can't post screencaps of my NotSo settings in Winamp...
Master Volume : 0 - 200
Vol = 110
Main NES Channels:
Square 1:
Vol = 165
Pan = L,45
Square 2:
Vol = 175
Pan = R,85
Triangle:
Vol = 125
Pan = L,45
Noise:
Vol = 135
Pan = R,15
DMC:
Vol = 160
Pan = L,15
FDS:
Vol = 150
Pan = R,80
VRC6 Channels:
Extra Square 1:
Vol = 140
Pan = L,40
Extra Square 2:
Vol = 165
Pan = R,85
Sawtooth:
Vol = 115
Pan = R,15
MMC5 Channels:
Extra Square 1:
Vol = 85
Pan = L,45
Extra Square 2:
Vol = 150
Pan = R,75
Voice:
Vol = 100
Pan = R,20
N106 Channels:
Channel 1:
Vol = 165
Pan = L,45
Channel 2:
Vol = 180
Pan = R,75
Channel 3:
Vol = 175
Pan = L,45
Channel 4:
Vol = 180
Pan = R,75
Channel 5:
Vol = 140
Pan = L,45
Channel 6:
Vol = 165
Pan = R,70
Channel 7:
Vol = 110
Pan = L,45
Channel 8:
Vol = 165
Pan = R,70
VRC7 Channels:
Channel 1:
Vol = 75
Pan = L,45
Channel 2:
Vol = 150
Pan = R,80
Channel 3:
Vol = 120
Pan = L,45
Channel 4:
Vol = 150
Pan = R,75
Channel 5:
Vol = 155
Pan = L,10
Channel 6:
Vol = 135
Pan = R,70
FME-07 Channels:
Pulse A:
Vol = 150
Pan = L,35
Pulse B:
Vol = 150
Pan = R,80
Pulse C:
Vol = 155
Pan = R,80