Whether or not you hear a difference between the two modes of operation on a given amp like the Plinius is going to depend on several factors. The first is how much power your speakers require to generate your average listening levels. This is directly related to impedance (because what we're really talking about is current and how much is required at a given frequency), and to a lesser extent sensitivity. The next factor is your room size, and then the third factor is how loud you typically listen.
In a small room, on efficient speakers, if you tend to listen in the high 80's or low 90's you may in fact never leave class A when in the sliding bias mode, hence you don't hear a difference between the two modes. However if you throw on a very dynamic piece of music, say Mahler, and play it full tilt boogie in a large room on inefficient speakers... well that's another story altogether. You will likely either be out of class A entirely or constantly at the bias point, switching between Class A and Class B (which is the worst possible scenario sonically speaking)
Now as for comparing two different amps from the same manufacturer, that's another story and it's hard to say whether a different amp will sound like the one amp in AB mode, or completely different.
In a small room, on efficient speakers, if you tend to listen in the high 80's or low 90's you may in fact never leave class A when in the sliding bias mode, hence you don't hear a difference between the two modes. However if you throw on a very dynamic piece of music, say Mahler, and play it full tilt boogie in a large room on inefficient speakers... well that's another story altogether. You will likely either be out of class A entirely or constantly at the bias point, switching between Class A and Class B (which is the worst possible scenario sonically speaking)
Now as for comparing two different amps from the same manufacturer, that's another story and it's hard to say whether a different amp will sound like the one amp in AB mode, or completely different.