Most regular box speakers are rated at 8 ohms. Receivers are designed to handle this relatively easy load because most people buy a few hundred-dollar 5.1 box speaker setup. These box speakers can swing up or down in impedance but on average stay around that 8-ohm rating. Martin Logan speakers can swing down to 1 or 2 ohms when there are difficult high frequency / detailed passages. If you look at the specs of the ML Source, the impedance rating is 5 ohms, 1.6 ohms at 20 kHz. This dip in the impedance requires an increase in current to supply the speakers. MOST receivers are not designed around this fact that almost all people with mid to hi-fi speakers purchase amplification for their speakers (MOST receivers are the easy, main stream answer to home audio, thus why spend money to design something that will most likely not be needed) and will look toward separates (separates: have a dedicate pre-amplifier with volume control that sends the signal to am amplifier. This takes the amplifier out of the pre-amp box that usually leads to better sound). If you look at the ratings of good to great amplifiers, you will see that as the impedance drops the wattage or power to the speakers will / or will almost double (say 200 Watts at 8 ohms, 350 to 400 Watts at 4 ohms, etc.). They are designed to handle speakers of the same / higher caliber as themselves. This is a high level explanation for the response you are receiving thus far.
Basically, looking at your receiver you should not have a problem. You have a mid-level receiver and as long as you are listening at light to moderate volumes you should never have a problem. But, if you want to experience what Martin Logan speakers have to offer, I would recommend buying external amplification and connect it to your receiver via the PRE OUTS on the back of the receiver panel. Have fun for now, save your money, and get a solid $500 - $1000 stereo amplifier by Sunfire, Classe, Conrad Johnson, or even Emotiva. Ask questions and get advice, which is what we are here for.
