Audio Research's Foundations series is voiced to be a little more on the tubey side of things sonically. They give up some of the black background, resolution, and absolute truth that are hallmarks of the Reference Series, but also have a little bit more of a pronounced that sounds like a tube amp with a sense of lushness and romanticism that is lovely to enjoy. The Foundation series contains the LS28, the CD6SE, DAC9, and VT80SE.
Of all the Audio Research amplifiers, the VT80SE is the a sweet spot in terms of an amplifier that combines a good amount of power, a price point, and a classic tube sound. Sonically it is quite unique, being the first Audio Research Foundation series amplifier to use KT150 tube. This combines the lushness of the Foundation series with the sonic benefits of the KT150.
The KT150 tube was designed from the ground up to be an audiophile tube. It's much more powerful than a KT88/6550, but also a delicate sound with fine 'alive sounding' midrange similar to EL-34 tubed amplifiers. The KT150 construction design is considerably more robust, and and powerful than the KT88/6500 series which allows it to not only push to much higher power, but also longer life. The KT150 tubed VT80SE has a general tube life expectancy of around 3000 hours, up from 2000 hours for the KT88 tubed VT80.