Pros: Clear and Liquid sound, fabulous mids, small and portable, insane battery life
Cons: Expensive, No USB Charging
This is one of those times when you just have to have the most expensive toy on the block.
The P51 Mustang is one of those products that sit in the high end luxury category of its product range. It is without a doubt a premiere product, with excellent sound quality as a headphone amp. However, I would stress that the gains this product has at its highway-robbery price point are not commensurate at all compared to a similar product sitting at a much lower price bracket.
While this product has sublimely clean sound with excellent, liquid mids and tight bass, this kind of micro detail improvement is just not worth the $400 or so asking price. This is the kind of product that you just get when you have the money to burn, otherwise I'd recommend a lot of nice products at much lower price points that give the same functionality and comparable audio quality, and oft times more features, such as the GoVibe Petite, which is smaller, has swappable batteries, longer battery life, USB charging and a DAC to round out the package. I simply cannot justify getting a product like this when much cheaper products do pretty much the same job, and I would be wary of those who say it is a much better product than the more affordable ones just like it.
The RSA P51 is, as its name implies, the luxury car of portable headphone amps and its price and differentiation from other products of its ilk reflect it. It's something nobody needs, everyone wants, and when it comes to the bottom line, still does the exact same thing its lower-priced brethren does, albeit with a tad more style and pizazz.