Amplifying a bass, Ubass or otherwise, is all about moving air. In order to make it loud(er) you have to move a lot of air. There are two ways to do so. The first is with the volume of cone area - the bigger the speaker, the more air it moves. The second is with the travel of the cone - the further it can move back and forth, the more air it moves.
Most small, portable amps use fairly small speakers due to space limitations. This leaves you with the need for a longer travel in order to move a sufficient amount of air. Unfortunately, most of the are spec'ed with shorter travel speakers which more accurately meet the frequency requirements of instruments which play in higher registers than the bass. The ones which are capable of the longer travel are generally quite a bit more expensive due to the tighter manufacturing tolerances required, the result being that a small, efficient, good-sounding bass amp is generally as expensive as normal sized bass rig if not moreso.
This leaves us with the option of increasing the cone area, either by using larger speakers or more of them, both options which preclude smaller speaker cabinets. FWIW, the fullest, richest bass tone I've ever been able to achieve was with an Ampeg or Acoustic 8 X 10 cabinet. However, moving those things from gig to gig requires an amount of effort I am both unwilling to put forth or pay someone else to do.
As far as amplified Ubass sound, I've found them to sound extremely full and detailed, given the proper amplification gear. At shows where I've used mine with electric cigar box guitars and even a when performing with a heavier alternative band, comments from audience members have provided similar observations. As a matter of fact, my little Rumbler has literally stolen the show a few times even when sharing the stage with some of the finest handmade custom CBGs.
