As the above post mentions, once your carbon filter is fitted the 2x 140mm outtake fans will probably lose a lot of their 'pull' due to having to pull air through the carbon rather than having no impedance. Then you will end up with air coming in quicker than it can be filtered out, so you will get positive pressure (not good) and smelly air finding its way out where it can, rather than through the filter.
Turn the 2x intake fans off and that can't happen as air is only being sucked in as fast as it is going out.
I set my stealth cab up pretty similar to yours when I first did it, but I used a 5" pc server fan in a hole at the top, and I made a carbon filter out of a small cardboard box - (really ghetto but it does actually contain the smell. I just change the carbon after every second grow). That is why I know the fans will lose performance. It still worked but it was a REALLY noticeable difference between filter on and filter off performance. I have since altered the cab and use a 4" extractor fan I got for £20 which performs much better with the box filter.
Your cabinet looks much better built than mine, you are obviously good with diy, but I have a question - is that silver duct tape you have on the outside? It looks good but you must have used a full roll lol.
