Just before I left for a holiday in Australia late last year, I found out I was to be receiving a pair of Sennheiser HD-700s for evaluation. I had, not long before that, purchased a Fostex HP-P1 to take with me as a high quality DAC/amp to use there. As most of my evaluation time would end up being while I was in Australia, I was eager to get a good amp to go with it. By chance, a fellow Aussie was selling an L3, so I made arrangements for a friend to receive it for me so I could pick it up after I arrived.
This plan turned out to be a good one, as while the Fostex is quite a decent DAC, it isn't as strong a headphone amplifier, something which the L3 made up for in spades, bring the usual results of better amplification -- a more expansive soundstage with better instrument separation and more detail. It turned my portable rig from good to excellent (and expensive to very expensive) and made listening very enjoyable. It drove the headphones I have with authority, but sounded somewhat deliberate in doing so, compared with my main amps which seem to do so effortlessly down the most subtle nuances. However, despite this, with a good source -- either the Metrum Octave or Fostex HP-P1 and my Symphones Magnums, the L3 conveyed the feeling of the music so well it was simply a joy to listen with.
I did experiment to see how much each device affected the chain, and the result was that the L3, directly from my iPhone, wasn't so crash-hot.
The design of the amp, with rear RCA jacks suggests it may be just as good at home as a main amp as a transportable one. With the power supply plugged in the character of the amp changes slightly, very similarly to the subtle change caused by the gain switch. However, with the front connection, if you are a fan of using those (awful IMO) Viablue plugs, you're going to have issues plugging them in, as the region of the metal case where the input and headphone out are is recessed slightly, and the recess may not be big enough for the largest plugs available, such as the ones from Canare.
Also, it's a relatively large and heavy amp for a portable, weighing in at 445 grams with 2 9V batteries inside. Compared to the Fostex HP-P1, it's about 1cm longer, a touch wider and somewhat thicker, so once you add an iPhone or iPod, you end up with quite a heavy and chunky rig to carry around. For this reason I more consider its use as a transportable rig to take with me when travelling away from home -- not for use on transport but after I've arrived.
The iBasso DX100 closes the gap with the L3 when it comes to headphone amplification, especially as it is a single device, but the L3 was still slightly better (though arguably not ~$800 better) and is also a better source IMO than the HP-P1.
My main headphones for travel at the moment are the Symphones Magnums V4. I felt very lucky after getting the L3 that I'd essentially scored a good component match with my entire rig. I preferred the results with headphones that already have a significant bass response -- the HD-700s and Audio Technica W3000s I didn't like as much paired with the L3. Those headphones sound more pleasing to me out of the hybrid Stacker II, the tubes of which present a "warmer" sound with a bit of a boost in the bass. I don't feel that the L3 introduces any particularly significant coloration to the sound or is lacking in any particular frequency region, more so those two pairs of headphones were coloured in the wrong direction for me with the wide variety of music I listen to.
The L3 includes a bass adjustment control which can add a bit of body to the sound. I usually leave it switched off as, gentle as it adding bass, even where I have felt a desire for a bass boost, I don't feel it has helped.
Ultimately, neither the HP-P1 nor DX100 are going to replace my main DACs and the L3 isn't going to replace any of my main amps, but both a good job of being very competent while being a firm step behind my home rig of Audio-gd Reference 7.1 and Phoenix. Unfortunately I haven't had a chance to compare the L3 with the new balanced amps from RSA and ALO, but I hope I get a chance to do so in the future, as that will be an interesting comparison indeed.