The situation:

-Windows 10 PC running Tor Expert Bundle on IP 192.a.b.c
-Windows 10 PC running DarkMX 1.3x on IP 192.a.b.d

Everything is up and running without any issue, except for one.  Creating a new (chat) channel always returns "error: couldn't listen for connections: Connection refused (10061)". Is there anything I need to add to torcc ?

CookieAuthFile (MyPath)\Tor\control_auth_cookie
CookieAuthentication 1
CookieAuthFileGroupReadable 1
ControlPort (a port)
SOCKSPort 192.a.b.c:(another port)
SafeSocks 1
SOCKSPolicy accept 192.a.b.0/16
SOCKSPolicy accept6 FC00::/7
SOCKSPolicy reject *

with external client can you connect to other channels?
with external client can you download/upload to others?
can you make a channel if you use the build in TOR client?

1. No problem
2. No problem
3. Too complicated because of local network layout. Also, no matter the outcome, it won't solve the issue with external client.

No change with v1.37, which is as expected since the changes were elsewhere in the code.

Connection refused is probably returned when it tries to TCP connect to the control port on the Tor machine.

1 - Can other users download files from you?  I suspect not.
2 - Go into Help > Diagnostics > System Log.  Any interesting lines in there?
3 - In the administrator command prompt on the machine running Tor, type netstat -ab and verify the control port is active.
4 - From the DarkMX machine, can you telnet to the _control_ port on the Tor machine?

#4 is the real test.  I don't think that control port is reachable, for whatever reason.

1. No idea.
2. No clues in there.
3. Port is active.
4. Can not telnet to control port (PuTTY returns a "connection refused")

I have no idea why I can't reach the port (even without an active firewall).  Logs show nothing unusual.  Searches for info about special config requirements in torcc when used with DarkMX also return nothing.

To use an external client, I don't think any changes are needed in the default Tor config files.

But it doesn't matter anyways, if you can't telnet to the Tor control port from the same machine running DarkMX, then that's the show-stopping problem right there.  And DarkMX erroring with 'connection refused' seems to be correct if your telnet client is reporting the exact same error.

In short:

DarkMX can't use an external Tor daemon at it's full extend because it only allows connections from localhost (127.0.0.1) on the control port.  Sure, you can connect and chat and download files but creating a new channel or uploading shared files is not possible.

In older versions you could set the control port IP in .torrc but, unless I missed something, this setting became obsolete.