"Loudness" is nothing more than having both the bass and treble set a little higher than zero. If you turn up both bass and treble a tad, you really have the same thing. Having two knobs allows more control than a single button, so you haven't lost anything--you've gained the ability to customize it more.
I have a slightly older version 8211 which looks almost identical with the same specs and it has a selective tone control which is sort of like the loudness button. Does the 8255 have the selective tone control button ?