Given my perception that the intent of the Pico Power is to be something of a do-it-all portable amp I used my Etymotic ER4Ps, Grado HF-2s and Sennheiser HD600s throughout the evaluation. I used my iPod Classic as the source with mostly lossless files on tap with genres covering a very wide range. It was with a smile on my face that I flipped the power switch, experience a short snap as the amp powered up and then was greeted with was blissful silence, which as I've stated in past reviews is exactly what I expect when I turn on a professionally built piece of audio gear. Double that expectation on a portable device that it likely to be used with highly sensitive IEMs. That's not to say that if you crank the volume pot to max on high gain with no music that you won't get a little hiss, you will, but if you're listening to IEMS (or most anything else) at that point it won't matter, you'll soon be deaf. In the usable volume range on all three headphones that I tried there was nothing to be heard even without a source connected. Channel matching was also superb, staying equal all the way down to undetectable levels, which is something that plagues many a portable in my experience.
Once the music started the news continued to be good. Honestly, I'd forgotten how good the Etys can sound driven by something other than directly off whatever MP3 player I'm toting around. Bass was nice and deep, punchy and very articulate. I know folks used to gripe about the ER4Ps being bass-shy, but they really do wake up when driven. Mids and Highs were clear, unsplashy and very neutral. Vocal sections were rendered with great detail and speed. If there was distortion on a track it was there, rendered in all its unfortunate detail.
If it sounds like I'm having a hard time finding a fault it's because I am. This is a fine bit of kit. With the HF2s, which are pretty neutral for a Grado, I was impressed by the amp's ability to render high frequencies without getting harsh. However the real surprise came when I cued up one of my favorite demo tracks with my recently re-acquired HD600s. I'd simply forgotten how competent these headphones are. While clearly not as bright as the HF2s the 600s snapped to attention with crispness that they simply lack when driven by my transformer-coupled home amp (a DIY creation). Even with the HD600s I never had cause to use the amp on anything other than the middle gain setting but it's nice to know that if I picked up a pair of orthos I'd still have the necessary headroom to take them with me on my next trip.
So how is the amp to use everyday? I won't say that I've done that for the last month but I have been using it a lot. I don't have any real complaints but I do want to point out a few noteworthy details. One, the front panel is very compact (duh?). There's enough room for everything so I don't expect users to have issues unless they have some absurd custom 1/8" plug on their LOD or headphones and in that case it's an issue that would occur almost regardless of amplifier choice. The important bits are accessible, which is what matters.
Two, the volume pot is very stiff. I don't know if this will change somewhat over time but I found it far more blessing than curse. It's easily adjusted and actually helps prevent accidental volume changes while in use. But it's not something I've encountered before so for that reason alone it was noteworthy.
Three, removing the batteries is a simple affair but one best suited to a stationary, stable location. A portion of the bottom panel is removed using two knurled nuts while the studs are semi-permanently installed. The cover slides rearward and then comes off. Reinstallation is the reverse and I found thumbnail tight on the nuts to be more than enough to survive walk-around and airplane travel duty. That said the nuts are tiny, pencil-eraser sized jewels that if dropped on say a subway would disappear quickly.
Four, both the gain and power switches sit nearly flush with the front panel. Again, way more blessing than curse as it takes a decisive action to change the switch position. The only time this was remotely an issue was turning the amp on or off in pocket when a flight attendant wanted a word.
Overall the Pico Power is a highly talented, multi-faceted portable that isn't likely to leave an owner wishing they'd packed (or purchased) something else. It's small enough to pocket, has enough battery to easily survive a coast to coast flight with layovers and a week worth of nights in a hotel and is powerful enough to allow you to plug just about anything into it. So if that sounds like what you're after I don't think that I can give a higher recommendation.