Last summer, in an uncharacteristic fit of wanderlust, I took an American Airlines flight to London. Two days later, I rode the Eurostar train to Paris in the company of my daughter and my wife, a travel agent, who had secured first-class train accommodations on her professional discount. Our ride was brisk, but the upgrade would have been a waste at any price: The Eurostar food was vile.

Whereas the Pass Labs preamplifiers are designed by Wayne Colburn, the power amplifiers are the work of company founder and high-end audio veteran Nelson Pass, who even lays out his own circuit boards. The X-model amplifiers, beginning with the X1000 in 1998, were the first implementation of Nelson Pass's patented Supersymmetry topology (see "Nelson Pass on the Patents of Pass"). The XA series, which debuted in 2002, combined Supersymmetry with the single-ended class-A operation of the Aleph series. The XA.5 models offer detail improvements over the XAs.