Ultra portables are as follow:
Ibasso T series (T3,T3D, T5)
All of them are Pretty Tiny....
I have owned all 3.... just sold my shadow yesterday. Have a Pico slim and the T3 was stolen.
RSA shadow - great amp, digital volume control.... great build quality. Slightly Warm which pairs well with colder gear. Adds just a little big of body to the music...a little more full. Battery life was great....more musical
Pico Slim - a bit on the cold side.... made things a more quick... decay was a little shorter. didnt add as much body as the shadow but drives things really well....not as musical but better for crital listening.
T3- pretty neutral.... slightly more analytical than the other two. performs well for the price....but definitely a couple steps down compared to the slim or shadow.
UHA4 - great amp, kind of like a mini alo rx or headstage arrow.... analytical, cold, quick.... has a dac so thats a plus but had a funky size....its wider than most amps and a lot shorter. it was more squarish than rectangle.... if it was more narrow and a little longer....it would be a very hard amp to beat for its price.