Mine is at least 50 open in the back and 95% open in the front assuming the door is removed which takes about the same effort as removing a grill cover. I would have reasonable clearance on the sides as well.
All kinds of cabinets but thanks for clarifying your specific set up. If you are removing the font when in use than you can go with one fan. I'd still mount it as high as possible on the back of the cabinet (or set the fan on the shelf behind the amp if possible) but instead of pulling hot air out the back I'd blow it into the cabinet and blow the cool air across the amp. You do not want a lot of air flow as the tubes do need heat to work properly. You just don't want the heat collecting above the amp.