I think its obvious that the main way this site gains members is those who go to the private section on the poker sites and see the game listed. The best way to increase new members would be to find some way to get people to go to the private section on merge or whatever site the tourney is on.
Im not saying this is an easy thing to do but there might be some way of doing it. If you go onto Carbon you see that sometimes they get like 3K entries in their $50 freerolls. People playing those games with 3K entries for a prizepool of $50 would jump at the chance to play our games with ~90 entries but pretty much an equal prizepool. The thing is Carbons freeroll is on their main tournament page and that is where all the traffic is.
You have already spent money on advertising, why not get the list of entries into these $50 carbon freerolls and send them all 1 coupon. Count the cost of the coupons sent as an advertising expense, and it has the added benefit of being sent directlly to your target audience