The Inward Connections Vac Rac 4000 is a signal processing module system that consists of spaces for four different modules. It's much like a 500 series rack, but it's got it's own format and I believe only Inward Connections has modules that will fit it. The casing takes up three spaces in a traditional rack space, with it's own vacuum tube power supply, as all of the modules for it are indeed vacuum tube based. Each also has it's own connections, so it's not like a 500 series rack where you'll have connections built into the casing. There are four different modules to choose from that fit the Vac Rac 4000, including the TEQ-1 Tube Step Equalizer, TMP- 1 Tube Mic Preamp, the TLM-1 Tube Limiter, and TII-1 Tube Instrument Interface. The rack that I've used had two of the TEQ-1 Tube Step EQ's as well as one of the TLM-1's and one of the TMP-1's, giving you the ability to have a really nice all in one chain. The studio that I work at has this, but it isn't always at the studio as the owner uses it at his home studio a good bit. I was really excited to get to try it out recently when he brought it in, and was not at all disappointed! These modules sound incredible and the Vac Rac 4000 is really a great idea from the smart thinkers over at Inward Connections. It's the only rack of it's kind that I'm aware of that actually lets you mix and match vacuum tube based gear. Inward isn't all that well known, but they make some of the best gear out there when it comes to finely made devices. This is definitely expensive, but if you're a professional looking for some tube gear that allows you to mix and match modules, the Vac Rac 4000 is the only way to do it!