It doesn't affect the sound as the output drivers in the empeg are quite capable of driving low-ohm loads (100 ohms or so) - but it does discharge the output caps a bit quicker on power off, which is where the thud prevention comes from.

Normal in-car equipment (and the mk2 empeg) has both a permanent battery line and an accessory feed: when you turn it off, it actually stays powered up for a few seconds after turning the amps off in order to prevent such thuds.

Hugo