While working in my original solo case last night, I noticed that there are ventilation holes below the fan on the back panel. They are adjacent to the PCI slots as can be seen here:http://www.silentpcreview.com/article272-page2.html
The newer Solo II has them too. Is this common in other cases? It seems silly to put those filters on the front of the case when you may be pulling unfiltered air in through the back panel. It makes an argument for using two fans on the front panel and slowing down the rear panel fan to make sure that you keep positive pressure within the case. I'm thinking I may want to experiment and block them off and see if my temps get better or worse. I imagine they would get worse.