Since you're listening in stereo you should first make sure the source component, i.e. the cable set top box or DVR or TV iself, is set to output its sound in stereo, not in surround sound, i.e Dolby, DTS, etc.
Next, be aware that in stereo you can use the Denon's tone controls or graphic equalizer. From the Treble tone control's default position turn up the Treble about halfway between the midpoint/default and the maximum setting. This will make the dialog stand out more. If there's an equalizer try boosting the 1KHz and 3KHz controls a notch or two to emphasize middle tones to enhance the voices. At home we have Magnepan speakers which are similar in tonal balance to your B&Ws. Music is played mostly on Windows Media Player or VLC both of which have a multi-band equalizer. TV and movies are played with Windows Media Center which has no tonal adjustments (oh, darn!) so we turn up the Treble control on our stereo amplifier. We actually gave up on 5.1 because the center channel dialog was too often not clear enough for our ageing ears, and when a receiver is in Dolby Surround mode you can't adjust the tone.