It is important to be aware of the massive irony of Willy mentioning Dave Singleman just as he is about to be fired. Dave Singleman represents another important mythic character in the play who exerts a curious fascination and hold over Willy. The way in which Willy looks up to Dave Singleman as a model of what a salesman should look like is clear and obvious from the way in which he talks about him and the description of his funeral:
...when he died, hundreds of salesmen and buyers were at his funeral. Things were sad on a lotta trains for months after that.
However, Willy is blind to the fact that Dave was one of a kind, a "Singleman," as his name suggests, which indicates that his prowess as a salesman cannot be repeated. This is another example of how Willy picks the wrong heroes for his life and the wrong people to aspire to modelling, just like he picks the wrong dreams.
Dave Singleman also highlights the contrast between Willy and Howard. For Willy, being a salesman is about something larger than just figures and profit; it is about personality and status. For Howard, all that it is about is how much you sell. Ironically Willy is trying to use an argument that will fall upon the deaf ears of his audience.