I'll second Nova's opinion though.
If you have a Rat Shack SPL meter, you can either use its own omni-directional mic. All you have to do is to plug an RCA wire at PHONO'S OUT (on SPL) and to the Denon's AUXILIAR port - LEFT CHANNEL.
Than go to the menu setup and select LEFT CHANNEL and you'll be ready to go (manual doesn't mention that, though).
You'll have pretty much the same results as if using the original Denon mic., with the advantage that having an SPL meter, you can do many, many more
adjustments, not mentioning that it'll cost you less $$.
Cheers.