I also wanted the HPA4 so bad but changed my mind after reading this comparison review and no regrets.
My Video Review: One of the biggest hypes I ever encountered in the headphone kingdom was the announcement of THX Achromatic Audio Amplifiers that revolutionized the way we listen to music. By far, the biggest impact on the market had the THX-AAA-789 by Drop. The company that made it, prepared a...
soundnews.net
As a headphone amp only, can't get any better than the A90. If you need a stellar pre amp with headphone amp, then the HPA4 is the way to go.