Dan D'Agostino judges an amplifier by what he can hear behind the singer. On an old Billie Holiday recording he listens past her voice to the band, and he told Jason Victor Serinus of Stereophile that with his Relentless amplifier he could hear the musicians breathing. "That's the stuff that can give you goosebumps," he said. Alan Sircom of Hi-Fi+ writes that Dan considers "abundant" power necessary for realistic sound. His amps double their output each time the speaker's load halves, so they keep up with speakers that are hard to drive. They look the part, too, with copper heatsinks whose holes Michael Fremer compares to a carburetor venturi, drawing cool air in from below. Ken Kessler of Hi-Fi News finds the M400 MxV as "ear-friendly" as the best tube amps. The brand suits owners of large, demanding speakers who want music to stay easy at any volume.

Kessler sorts the three lines the way a supercar maker would. Relentless is the limited-edition flagship, Momentum the grand tourer, and Progression the luxury SUV, the most practical to own. Progression is the place to start. Sircom found the Progression pre and power amps so close to Momentum that "their biggest rivals are their little brothers," and he thinks Progression is more than enough for most people. The step up is Momentum. Dan told Serinus that if he could own only one of his amplifiers it would probably be the Momentum, which in his words gives the most of his sound for the least money. Match the amp to the speaker. Kessler judged the S350 overkill for high-sensitivity DeVore O/93s, while his Wilson Sasha DAWs took to it "like gin to tonic." Plug it straight into the wall, as Dan advises. Serinus usually uses a battery regenerator but preferred the M400 MxV without one. Get help with delivery, since a Progression S350 weighs 115 pounds and has no handles.

Dan D'Agostino studied electrical engineering at UC Berkeley and served with the U.S. Navy's Underwater Demolition Team, and in 1980 he co-founded Krell. Sircom recalls that the Krells everyone wanted in the 1980s were the KSA-50, a pure class A design, and the more powerful KSA-250. Dan and Petra D'Agostino started the new company in 2011, and its first product was the Momentum monoblock. Fremer found it remarkably compact for its power, with thick copper side panels on a chassis machined from one aluminum billet. Dan collects watches and loves wine and cars, and Fremer saw all three in the design. He first heard Momentums at a CES, driving Wilson Sashas, and described the sound as "ultra-transparent and velvety rich." The Momentum's success led to Progression below it and Relentless above. Every unit is built by hand in Cave Creek, Arizona. Kessler has used Dan's designs for nearly four decades and keeps a Momentum Stereo as his solid-state reference.