Question

I would like Studio One to remain OPEN when closing songs using the red x button...
...It should be the same as when closing using command-W: Studio One remains open.

Answer 1

You can use File > Close Window

The Red X is close program across all OS X software if they follow Apple's guidelines.

Answer 2

(to GLJones)...you said....

"The Red X is close program across all OS X software if they follow Apple's guidelines".

??

...so (Apple) Safari, (Apple) Numbers and many others don't follow Apple's guidelines??....

...It is NOT always the case...