Hi again, Joe.
Now to answer your questions.
I use the Native Instruments Audio 2 sound card. It has an 1/8" connections (one for master and one for the headphones). I'm wondering If I can use this stereo 1/8" to xlr cord to go into 1 channel of my T8S.
Well, I wouldn't use that cable because it shorts the two outputs from the soundcard together.
"Signal Transmission:XLR to 3.5mm jack cable combines the left and right stereo signal from a source with 3.5mm stereo output into XLR Single Mono Feed. " -- Source
See Stereo to Mono Conversion
And If so will this make the signal mono.
I'm trying to keep my computer on one channel so I can easily adjust the gain when doing intros and announcements at my gigs.
If you use the T8S USB B from PC instead of the soundcard, you can direct both left and right channels to a single channel on the T8S (e.g., channel 8). If you did that, you would have both channels mixed down to mono and one volume control for that.
or would I be better off getting one of those Big Knob mackies to utilize as my sound card and have the one big knob available to control my computer volume?
Do you want to have another piece of gear at your desk?
I'll be back with another idea shortly.