Four things!!!!
1. I can't believe that thing is legal to sell! there can be as high as 300 volts going into some of those pins!
2. I am certain that it will add noise to your amplifier from being able to place the unit nearer things it shouldn't be; among other reasons.
3. It is a bit overpriced by A LOT. There are likely only about 20 components in that thing from input to output. That box probably could be made at the homebrewer level for about $50. I wouldn't pay more than $200 for it.
4. There is the potential to install this into the VERY WRONG tube socket. IT probably won't damage anything, but it will certainly not work if you place it in the Reverb, Tremolo, or Phase Inverter sockets. What, if any damage that can occur, is up for debate. So as an end-user, you will have to be pretty familiar with your amps tube socket layout. Most are not that savvy. If you can't bias an output tube, you probably shouldn't be using this without supervision.
Don't get me wrong, I'm sure it does what it says ( adds another two stages of gain ), and it probably sounds pretty good even, but I see it as a gimmick. It is not much different than placing a dirt pedal in front of your amp. A different approach, same thing.
