So if it is a limitation of the Denon receiver itself, then an outboard phono stage will be of little help, and if replacing the Denon is off the table (as you mentioned previously), then finding a cartridge that's a better match for your current system is the best option. If you have a dealer you can take your system to, he could let you hear different options throughout the playback chain, but without being able to do that, you're left to take your best guess and see what happens.I compared them... I like the Dennon's phono better.. but, i ca't tell if it is for the what it actually is, or is that their line inputs are not catching all that analog smootheness.
So, just can't solely blame the mmf indtegrated one.
WizardWG4 mentioned capacitance as a possible problem. The Ortofon OM series has a wide recommended capacitance window of 200-500 pF, as opposed to AT's recommendation of 100-200 pF. You might want to add an Ortofon OM to your list of possible cartridge options.