Stereo Tool

I have mAirlist 6.3 as software. Can someone tell me which version of StereoTool and which DLL I should use which one?

A quick google looking at what kind of plugins are supported came up with a Stereo Tool specific wiki page: https://wiki.mairlist.com/config:dsp:stereotool

It appears the Winamp plugin is the one you would want to use.

Yes, https://wiki.mairlist.com/config:dsp:stereotool that's a good start...

I decided to give mAirList a try to see what is going on here...
Downloaded generic_stereo_tool.dll to try it.
Copied the file to mAirList 6.0\plugins folder as directed.
What I did figure out is that current ST Winamp plug-ins don't work, fact is the plug-in they recommend doesn't work in my Winamp! So I'm not so sure of it's version.

To make the plug-in they provided work I went to the icon mAirList (on the top left or use CNTRL-ALT-X) to access the Control Panel. After a little toying with it I found that these setting made it work for me...
[Audio Device] Default playback device: Encoder (Redirects audio to the Encoder)
[Audio Device] [Playlist 1] Player 1 Playback: Encoder
[Audio Device] [Playlist 1] Player 2 Playback: Encoder
[Audio Device] [Encoder] Playback device: DirectSound: Speakers (Choose your audio out source here)

[Encoder] [Sound Processing] [Master] Add... Stereo Tool > Choose Configure
Selecting "Advanced " Is where you can enter your StereoTool License Key (If you have one) You don't need to add to each player if you configure it for the Master. (This was just my preference, esp for testing)
IMO, I think if you want to use a current version of ST with this you might want to consider to use a Virtual Audio Cable (VAC) to a standalone version of ST and from there to your final output.