The lower the resistance gets the more the load starts to look like a short. Think about it, if you took a piece of wire, with a resistance of virtually 0 Ohms, and put it across the speaker terminals, the receiver would shut down. Lower resistance isn't always better. What you want is a load that's compatible with what the receiver is expecting to run into.
Running speakers in series will present higher resistance, and won't make the receiver act like it is shorting out, but speakers aren't fixed loads like a simple resistor. Speakers are reactive to the frequency they are reproducing and can run from less than 2 Ohms to over 20 across the audible range. And different speakers react differently at different frequencies. So yes, this will have an effect on the sound.
