Thanks for the suggestion, but I think the Pico Slim is meeting all my portable amping needs. I was fairly happy with the SR71B when I had only the JH16 and ES5, but the 71B's gain was too high for certain sources. The 71B also made those customs sound a bit overpowering in the bass, especially since the 16 and ES5 seem to have an inherent bass presence to them irrespective of source or amp. I mainly got the Pico Slim to get better volume control with the ES5, but it turned out to be a great match with the UERM.
I just pulled out the TTVJ Slim. It's been a while since I listened to it, because it's too loud even on the lowest volume with all of my customs when using the CLAS (my main portable source). The TTVJ reveals a bit of hiss with my customs even when it's not connected to anything, and the volume pot is only 1-2 notches above silent. Meanwhile, the Pico gives me a dead silent background at all volume levels that a human could stand to listen to.
Compared to the Pico, I'd say the TTVJ is more laid back. It has a sort of faded vinyl feel to it, which could either be dull or natural depending on your preference. I'd say that the TTVJ holds a bit of the UERM's treble delivery back, while emphasizing the midrange and mid-bass. On the other hand, the Pico sounds more polished and "digital" to me, although I imagine some could find the UERM-Pico combo a bit harsh or analytical. I don't--the Pico's focus and balance feel natural to me. I think the Pico edges out the TTVJ in terms of imaging and soundstage dimensionality as well.
Bottom line: I listened with the TTVJ for about 20 minutes. But once I put on the Pico, I just felt closer to, and more immersed in, the music. I had no desire to switch back.
I was surprised to read that Joe found the UERM's treble harsh and even sibilant. Although I feel like the UERM's treble is more crisp than either the JH16's or ES5's, to me it hasn't sounded brittle or strident. Whereas the 16 and ES5 could sound a bit shy in the treble region, the UERM seems to remove a bit of grain from the treble. I'm sure that the prominent treble, coupled with the lack of a bass cloud over the sound, help contribute to the sense of clarity that I hear from the UERM. But I still don't think the UERM's technical ability is just a function of frequency tuning; somehow it sounds more capable, more effortless, and just more real than the other 2 customs I've heard.