Now, it's true that a person can overcome this tendency with a strong attention to what they're hearing. If you realize what the speaker is doing, and you admit that you have pre-existing feelings changing your judgement about their words, you can account for this fact; you can consciously ignore your feelings and focus on the logic of the speaker's argument. However, this takes a lot of focus and work to do. You can't just sit there and zone out, staring at the TV. You have to think about what you're hearing; look at what your feelings on the issue are, and then decide if those came about because of the logic of the speaker or the subconscious influence of your previous opinion of them.
So if you happen to be the speaker, how do you take all this into account? I would argue that depends highly on the context. For example, if you're outlining your latest breakthrough in electrodynamic quantum theory to a room full of professors and MIT students, you can be pretty sure that the audience is going to be thinking about your argument. They're probably active listeners that know about the topic already, and have an interest in breaking down your logic and actual argument instead of just writing you off without listening. However, if you're designing an advertisement to broadcast during a 3 minute break in the Superbowl, you can be pretty positive of the opposite sort of audience. The people watching your masterpiece are going to be excited about the game, probably slightly tipsy, and in no mood to think logically and deconstruct your advertising methods. In this case, their pre-existing thoughts on your company, as well as the subconscious suggestions you give them in the ad (hot women in bikinis, sports stars, people partying and smiling, etc.) are going to have much more of an effect, since the viewers aren't on the lookout to avoid such pitfalls.
I would argue, then, that as a listener we should accept Aristotle's argument, and do our best to separate our preexisting opinions of the speaker, as well as subconscious cues from their presentation, with the logic and validity of their actual argument. However, if we're the ones speaking, we much take all of this into account. We have to realize that we will never have an audience that is completely immune to such things. They may be more or less susceptible (such as a room full of scientists vs house full of beer drinking superbowl viewers), but in all cases there will be people listening to you that are swayed by things like prior opinion and subtle ploys to change their emotions.
