I bought this headphone amp after having met Ray Samuels at a Headphone show.  He made it absolutely clear how much he valued over-engineering his pieces and how much he prioritized long term reliability.  I've bought other Headphone amps since (A  FiiO E17 which is also a DAC).  It offers really stiff competition and it's probably better value for the money.  But I'm not sure if it will hold up like this P-51 has. (I will say, no complaints thus far.)  The E-17 charge lasts about fifteen hours and I've never been able to actually run this Samuels...

I use this Headphone amp with my V-Moda M-100's.  The 100's are just too "dark" without something to mitigate on the top octave's behalf.  The OLED display is bright clean and easy to read.  (A plus, for not a kid anymore folks like me.)   The charge lasts roughly fifteen hours, which is more than enough for most situations, but it's the one area where my Samuels P-51 reigns supreme.  (Well, that and maybe a just slightly more beautiful midrange.  I try to avoid terms like "liquidity", but the Samuels draws your attention to how good mids can sound...

The highs and the lows are, for lack a better descriptor "dry".  There are people who "like" this sort of leanness, but it's never sounded "real" to me.  It's the difference I hear in vintage (dry)Teac/Tascam analog tape gear vs. the better Pioneer machines from the mid-'70's forward. (The better European machines as well, even though reliability problems are legion, depending on brand.) How something measures and how it sounds are sometimes very counterintuitive.  I was disappointed and let's leave it at that.

I apologize if I duplicate the findings of others. I read the other reviews, and I'm trying to address the things I thought haven't been talked about yet.     The current (it changes, it has too I guess) price (HD7 version) is 29.00 USD w/free shipping.  (Musician's Friend, and a few other "musician" websites.  NO CONNECTION.)   I had to wait quite a while to get them.  The Presonus HD7 option would've been faster, cheaper and less hazardous to the merchandise.  At least potentially.....   I used the center "pads" from the Velour aftermarket...

I bought these as a demo from Music Direct some years ago.  And they're still on the front lines of my headphone stable.  They're non-fatiguing, and exhibit an openness no other headphone (even the 650, which I also love, but it's not the same cup of tea....) I've heard has.   I prefer these for Orchestral and Chamber music.  I have a set of Ultrasone Pro 900's that seem to integrate well with Big Band Jazz (Ferguson, Gordon Goodwin, Bob Mintzer, Buddy Rich, etc.) I have used these to master recordings of my Brass Quintet and they've performed very...

I think they're five stars at the price I paid. I might have to trim a piece of a star at the MSRP. My two references are the Sennheiser 650 and the Ultrasone Pro 900's, but these are preferable for portable applications and have virtues of their own.  They have an "immediacy" that I assign to their efficiency, I am encouraged to listen at lower levels on these 'phones rather than turning them up to try to get them to "enunciate" or bloom.  It's an unexpected virtue.  The Pro 900's hit me more like ear "speakers".  The 650's are where I get the most...