The ADL 300-G is a dual channel tube based direct input box. It is made up analog parts and each channel consists of 1/4" connections for your instrument input and an output for sending it to an amplifier as well if you so choose. It also has an XLR output. It isn't rack mountable as it is a large box to be placed on a desk or some other sort of surface.
If you are at all familiar with the make up of direct input boxes, the ADL 300-G won't present you with any new issues I don't think. Each channel simply has a knob for gain (which isn't something you see on many DI boxes), as well as switches for a ground lift and for choosing between thru tube and a direct output. As you can probably tell just from looking at it, it's easy enough to figure out how to use without needing help from a manual.
The sound quality of the ADL 300-G is top notch, as it has both a warm tube sound and will faithfully reproduce your signal. It doesn't add any of it's own noise or other sound, as what you send to it is definitely what you'll get out of it. Of course there is a bit of warmth added since you're going through tubes, but you can also go direct out if you don't want to use the tubes for whatever reason. I've used the ADL 300-G for recording bass guitar in tandem with a bass cabinet. It's nice to have the direct out to send your signal to amp, which although it found on most DI boxes, isn't always available. I don't have experience using the box as a stereo pair as of yet.
The ADL 300-G seems to be a pretty rare commodity, as the only one I've ever been in contact with is the one that I used. It is a bit surprising though as this is truly a great sounding box with a good amount of versatility for DI box. I find that it is very simple to use, although it will cost a bit more than average for sure. Having said this, you are getting two channels of a high quality tube based direct box, so if you're looking for something of this ilk, I'd suggest giving it a try if you're able to at all!