Hi bdog,
Wiring speakers in series is a no-no if you want smooth, linear response. It's true that some brands of receivers used this ploy to prevent the combined impedance of several pairs of speakers from falling to 4 ohms or less, but what may happen with series speaker connection is that the varying impedance curve of one speaker affects the frequency response of every speaker connected in series. Few loudspeakers have straight, linear impedance curves, so series connection of quality speakers should be avoided.
Foghorn's advice is excellent. In your case, go with the Denon over the Yamaha. Like H/K, Denons tend to have more robust amplifiers that will drive lower impedances without problems.
Regards,
