Any disadvantages to serial connections? I've got a DH500, capable of
driving 4 ohm, probably 2 ohm steady, and all my speakers are 8 ohm.
What will be the disadvantage of running two speakres in parallel for a
lower (but acceptable) resistance?

Electrically, the amp should cope. You might not get appreciably
greater volume, and your chances of a proper stereo soundfield will
diminish. If you mainly listen to the pan-potted mono that passes for
stereo in most commercial music, this may not worry you. :-)