To play DVD Audio, you must use the DVD Audio Player's 5.1 audio outputs. Your 7.l equipped Marantz receiver should easily accommodate the 5.1 worth of outputs. All you have to do is make sure you connect them correctly. (E.G., Center output to center input, subwoofer out to subwoofer in, etc...)
Your Marantz recognizes all the formats associated with regular DVD video discs like Dolby Digital, DTS, etc..., but these are strictly DVD video soundtracks that are carried by the digital cables (coaxial/optic).
Because of the music industry's copyright concerns, they've prevented high resolution multichannel recordings to travel digitally which, theoretically, could be duplicated digit by digit giving perfect copies. (Horrors!) Because the decoding is restricted to within the DVD audio player, you need the cables to transmit the signals to your A/V channel by channel. Hence, your need for all those cables between the DVD Audio player and the receiver.
Some DVD Audio players will "down mix" the multichannel high resolution DVD Audio disc to output two channel stereo. This could account for the A/V receiver's reading out two channels from your DVD Audio discs. You might try hooking up two old fashion RCA interconnects between the DVD player and A/V receiver and see what you get. Unfortunately, it won't be the sound of the new high resolution formats (24/96 plus).
Yeah, it was a drag buying more interconnects but it's fun hearing the new high resolution discs. Read DVD Audio reviews before you rush out to buy too many DVD Audio discs. Some older recordings reissued in the higher resolution format only reveal how poor the original recordings were. Some people get annoyed by a new multichannel presentation of a revered "classic". To some, it's like "coloring" black and white movies.
In short, not all multichannel discs will amuse you, some will astound, and a few will make you outright cringe. Read before you buy. Nonetheless, the higher resolution is a jump over the sound quality of the redbook CD's of the last twenty years. SACD and DVD Audio, the two competing high resolution CD formats, hold a lot of potential for the future. Buy those extra cables and enjoy the future.