The Valley Audio TR804 is a rack enclosure that cases certain Valley People outboard gain like the Gain Brain gates. It can host a variety of gear as far as I know, but I think it's only for Valley Audio gear, although I could be wrong about this. I've only used the TR804 rack with Kepex II Gain Brain modules, but I'd be surprised if you couldn't use other modules as well. Each can fit four modules and it comes with a power supply for powering everything up. This is very similar to the API Lunchboxes, but this doesn't have the same sort of flexibility that you will get with a 500 series casing since there are know so many 500 series modules to choose from. So unless you're going to get some Gain Brain modules or you already have some, you're probably not going to need the TR804. If you want something that you can build upon with different modules, a 500 series casing is the way go as these are not interchangeable due to the lack of flexibility with modules with the TR804. The price isn't too bad for the TR804, but that doesn't matter much as if you've got the Gain Brain/Kepex modules, you're going to need to regardless, but at least it's nice to know that you won't be getting gouged for something that is necessary. I've seen these in all different types of studios, as I think they were really more necessary before the digital age as now gates just aren't used for tracking as much. Still, the Gain Brain series of gates are some of the best around, so if you need a bunch of them, you'll need the Valley Audio TR804. I don't believe you can even use one of the modules unless you've got this, so it's essential for them...