+1 on the subjectivity factor. Loud compared to what, setup by whom, strung with what, played by whom, at what string heights? So many variables - strings, setup, and player's touch are equally important.
My fully carved "Deluxe" Upton Professor has the nice asset of even response throughout its range. I would not say, however, that it is loud. When played side by side with other basses, I do not find it noteworthy in this regard.
I might add that I think that my particular Upton, despite much fussing, is perhaps even lacking in acoustic volume. I have been through several string/setup combos since buying the bass a few months ago, so I have some empirical support to my (personal and subjective) opinion.
From what I have heard/read from several other Upton owners, most are playing one pizz style or another. Lots of rockabilly or jazz players. Most are of these are using amplification, as far as I can tell. When I was a working player, in a jazz context (small group or full band), I usually used a pickup/amp combo. I still do now, as an amateur.
In the trio instrumentation you mention, you may not need an amp with some basses, but I would not buy an Upton because you have the expectation that it will cut/push through the sonic stew in some unusual manner. That depends as much on the players, the room, and the above factors. Also depends on your playing style and goals.
I am in the process of working with the excellent local luthier/bassist Michael Hartery to improve the setup of my instrument and reduce unneeded weight/ballast. The point of the exercise is to increase responsiveness and sound volume. Highest praises to Michael's skill, insight, and strategic approach! If the instrument had been "loud" already, we probably never would have started working on it, however.
With the setup improvements Michael is making (endpin replacement, new bridge, new soundpost and soundpost placement, removing wood from the bottom of the fingerboard), my bass' volume and response is improving. Even so, it is still not an unusually loud instrument.
I played my bass and chose/bought it fully aware of its challenges. I was and still am fine with them - this is the realm of inexpensive instruments. With some additional setup work, my bass will be a reasonable, relatively cheap bass. It is adequate for my current, amateur purposes.
I did not buy my bass sight unseen, playing it twice before taking it home. I would respectfully and humbly suggest that anyone else consider using a similar process. IMHO, everyone should decide for themselves just how "loud" an instrument is.
