Features: The unit is a red box the size of a regular guitar pedal or a DI. It has a 1/4 inch "Line In" input and a 4x12 or "Combo" selector on one side. The other side is the "From Speaker Output" and "To Speaker Cabinet" 1/4 inch jacks. At the top there is a XLR Output which can power the unit via phantom power. If there is no phantom power the unit can also take power from any adapter from 6 to 12 volts. On the bottom of the unit there is a ground lift switch which is not labeled.

In Use : What the Redbox does is provide a DI box with speaker emulation so it can be used to either plug in a distortion pedal, an amp's line out or could be inserted between a speaker out and a real speaker. It could also be used on bass as a DI if you like to have a bit livelier "cab" sound to the bass. You have to be careful with tube amps as speaker has to be connected as well since the Redbox doesn't provide a dummy load like an attenuator would. I mainly use this for guitars when I get a band with a single guitarist and they need two guitar tracks or when I need the guitarist to remain quiet in the tracking room so there is no bleed on the drum tracks.
Some of the features are not well explained in the manual and I had to contact H&K about them. For one - the ground lift switch on the back of the unit that is not labelled. Also the way to hook up a tube amp to the simulator is not very well explained either. Again - easily clarified by H&K, apparently you can hook the speaker out of a tube amp to the "From Speaker Output" on the H&K Redbox and then run another speaker cable "To Speaker Cabinet". This unit is also lacking a DI output - it would've been very beneficial if you can get a DI signal out of it and your colored signal separated, in the present design you only get the colored sound out which limits the Redbox's useability.
Regardless of these shortcomings the unit provides a great sound and has 2 speaker emulations - one is a 2x12 and the other a 4x12. Both very useful and different in sound. The 2x12 sounds like a Fender or Vox amp - it is brighter in nature and a bit sparkly on the high end. The 4x12 sounds closer to a Mesa Boogie cab as it is darker and a bit airier.
So far the Redbox has worked great on sessions as a second sound option or as a single sound option when the amp in question is something so cheap that you rather not track it or if the guitarist brings in a digital fx unit with a harsh sounding speaker sim. I also use it to DI any kind of pedal that I have and can't crank up through an amp for various reasons.

Value: A little pricey for a little box, but there is nothing else quite like it on the market.

Sound: This product is a cab (speaker) emulator and it does the job well. It basically has two distinct sounds - one is 4x12 which to me feels like a cranked Marshall 4x12 stack and the other mode is a 2x12 which seems like a nice combo, say Fender or Vox. After owning the H&K Tubeman with Redbox simulation I decided to get teh Redbox alone to pair with some other effects for more tonal options and this thing rocks.

Ease of Use: This unit is pretty much plug and play. Some care has to be excercised not to blow up the tube amp (speaker always needs to be connected!) otherwise it is pretty simple to setup and use, and in cases where your situation doesn't allow loud sounds the Redbox is a must. Support: I contacted H&K on some questions I had regarding the manual - received answers within a day so that is outstanding in my book.

Overall: There's really nothing else on the market quite like it. I personally love the fact that H&K made sure that this unit can use all kinds of power supplies (from 6 to 12 volt!), phantom power or battery. Great design and great sounds.