I was just asked via PM my thoughts on the Pico Slim vs, the TTVJ Slim with the Beyer DT1350's. I thought it might be worth posting here:
Pico Slim is awesome with the DT1350's. If your plans include lower impedance and especially very sensitive headphones and especially multidriver IEM's, there is not a better amp in my opinion. The reason? The incredible volume control the Pico Slim has. I struggle with most amps, especially with my custom iem's, as one click is too quiet, and likely has channel imbalance. Either the next click is too loud, or by the time the pot moves enough to get past channel imbalance, it is too loud for me. Note that I usually listen more quietly than most seem to. I save the ear-bleed sessions for the car, or the speaker rig when the lovely wife and daughter are out. The Pico Slim is NOT a powerhouse. It is best thought of as a clean amp with a brilliant attenuator. It powers the DT1350's just fine, though.
If your plans do not include sensitive iem's, and any higher impedance headphones or less sensitive headphones, the TTVJ is also a great choice. There is just something about the TTVJ Slim and the way it sounds, that I really like. It has a warm, fun tonality to it that is really captivating. While larger than the Pico Slim, The TTVJ Slim is not much thicker, especially if you leave the Pico Slim in it's leather case. The TTVJ Slim also is a better shape with a DAP strapped to it, as it is the same length or longer. With the Pico Slim, you have a very small, slightly odd shaped box strapped to the back. Both are very portable, and both fit fine into a sweatshirt pocket or something.
So - Custom or universal multidriver sensitive iems - Pico Slim without question.
No iems - Either works well. Pico slim is much closer to neutral, wonderful volume control TTVJ Slim has a warmth and fun coloring that really sounds wonderful. Less sensitive / higher impedance cans - I would lean toward the TTVJ. Both the Pico and TTVJ Slims are excellent with the DT1350.