Well, my "last thought" was certainly premature. I did think of another reason that Yamaha uses a gradual volume taper for their new integrated amps. They are targeted for the audiophile market in spite of their low cost. The loudness control is actually quite thoughtfully designed, but in order for it to work the way it was intended, users need to be able to turn the volume down to sufficiently low levels. Thus you have a very gradual volume taper.
As far as the bass and treble controls are concerned, the two bands centered at 20Hz and 20kHz, apparently have a fairly broad band, so they control frequencies all the way up to 400Hz and down to 3.5kHz respectively. This would, again, indicate a very audiophile-like design.
When I master my recordings, the last thing that I do before printing them is to add either a Pultec EQ or in many cases, a Maag EQ. Both of these have the ability to add back fullness on the bottom and air in the top bands that gets lost in the mixing process. The bands that I use are typically 20 to 40 Hz bass, and 10 to 20 kHz treble. The idea is not to mess with the overall mix by shifting the EQ decisions that I have already made, but rather to add back low end punch and high end transients for sonic impact. Lots of phase cancellation occurs during mixing, especially in the 20-40 Hz range with kick drums, synths, and electric bass all competing for bandwidth. So the final mix can sound "light" in the bass. Same thing, albeit to a lesser exert with the transients centered way up high. So a sub bass and air band EQ fix that problem.
When you listen to a recording, you probably don't want to mess with the original EQ settings that the engineer intended. You might, however, want to compensate for your less than perfect listening space which may have too much or too little sub bass; ditto with the ultra high end. The low and high ranges on the Yamaha EQ are not going to muck up the recording, but will allow you to compensate for your room characteristics.