This is a brand new pair of mono blocks which include the latest upgrades, please check the website www.audiomirror.com. The whole amps are re-designed. They have a direct coupling between input and driver tubes, dynamic auto bias and protection for runaway tubes. First and foremost - about auto bias. This is the most recent and the biggest upgrade done. Sound dynamics and details are vastly improved. Power tubes run with a bias constantly following the signal and thus achieving very fast transient response, very well controlled low end and lowest distortions. It also eliminates the cathode resistor and the local feedback on the power tubes. This amps can reproduce the lowest ever heard bass from single ended triode amp and they have a scary fast transients. You don't need to bias or re-bias your tubes ever. It is done by the same auto bias circuit. Amps have a protection circuit which prevent from power tubes runaway, overdriving the tubes and over biasing. In the event of tube malfunction it will shut the amp HT and will show(with LED) which tube is bad. Email me if you have any questions at firstname.lastname@example.org For PayPal fees add 3%. Please visit our web site www.audiomirror.com 4th of July sale !!! Additional $200 off!