Sounds like your exhaust is too strong. I'm running a 131 CFM dayton with a can2600 for exhaust. My active intake is also a 4" w/a duct booster. This all housed in a 3.5'x5'x7' tent. So my tent is a little over 122 CF. You didn't include your height in your OP, so determining your total CF cannot be calculated. This has been my setup for the past year with no problems.
I still have a little bit of the walls sucking in, but not to the severity you're describing...just enough to let me know there is good neg pressure. Point being...can you turn down your exhaust anymore? With a 550CFM fan running at 35%, sounds like your pulling about 192.5 CFM. Including filter, duct, etc..., might bring it down to 170. Which still sounds too strong...depending on the total CFM of your tent.
An intake fan is a poor idea if you can avoid it. The exhaust fan will be limited by what the intake fan can push, which could possibly make it even less effective than what you already have, or your if your intake fan is too much you will lose negative pressure, and be pushing the smell out through holes in the tent. You need a much larger passive intake. A good rule of thumb is a passive intake 2x the size of the smallest point in your exhaust ducting/fan combo. This way any airflow loss in the turns and twists of your light trap intake will be minimized by doubling the size. So for instance if your fan is an 8" vortex, and your ducting is 8", then you will want a 10-12" round intake, or any shape and size that equals 100 cubic inches of area. You also need an passive intake into the room the tent is in that is the same size or larger, whether that is an open window or what is up to you.
The intakes are the main design problem I see with tents, someone needs to come up with some sort of ingenious high airflow light trap that could be built into or added onto tents. It is ridiculous that they sell tents designed for 1000 watt lights with 4 or 6 inch intake holes, that are not light proof.
btw greentrich, taking the rated speed of the fan and multiplying it by the fan speed controller setting does not get you to the actual cfms your fan is pushing. The rated speed of the fan is usually at a low static pressure. As the pressure ramps up, say by attaching a carbon filter, or any length of ducting, turns in the ducting, using flex ducting, a reduction in intake or exhaust size, all of these things add to the static pressure. A good fan manufacturer will have pdfs on their site telling you the stats of the fan, how many cfms at what pressure. These pressures are rated in inches WG, water gauge, for inches of a water column. Most fans will have lost significant cfm by 1", and be slowed down to just a fraction of their max cfm by 1.5-2".
Here is a chart of can fans: http://www.canfilters.com/fan_metal_home.html
You can see that the 8" can fan is rated at 483 CFMS, but that rating is taken at 0". Most can filters are rated at creating a .75" drop in pressure, so just having an 8" can fan hooked up to a can 66 brings the max speed down to 320 cfms. Now you add on a length of flex duct, 2 90 degree turns, an intake into your tent, and then the intake from outside into the room that the tent is in, and you could easily be nearing 1.5" As the pressure increases it is not a linear drop in cfms either, by 1.5" that same fan has dropped to only 153 cfms, and it maxes out at 2.16", with 2" not being even shown in the chart, probably due to it being abysmal. If you then take that 153 cfm fan and reduce it's power to only 35%, you will be moving about as much air as a computer fan.
So I'm guessing his 550 cfm vortex is pushing under 100 cfms right now, and is really working hard from all that pressure. Good ventilation is all about reducing the static pressure, and you have to account for every place the air goes through, from where it enters the house, to where it exits the house. You then work as hard as you can to minimize the pressure every step of the way, and first off eliminate the weak links.
I had the same problem with sucking tent walls.
I have a huge homemade tent that is in 1/4 of my 2 car garage. What I did was get 2 of those light dimmers switches at Home Depot. I turned my exhaust fan on a little, then my intake fan on. Watching my tent suck in and out, I adjusted till I had just a little more blowing in than coming out which pushes my walls out just a tad when my tent is closed.
What you have done here is introduced positive pressure to your tent. Smelly unscrubbed air is being pushed out of small cracks in your tent. It might be faint enough that you get used to it, living with it every day, but non smokers or someone who doesn't live at your place would probably smell it from a good distance.
I gotta say it again, it is almost always preferable to figure out where your airflow is getting bogged down, fix that, and use passive intakes, wherever possible. If you size it right it should not add enough of a pressure drop to be noticeable.
