Flat speaker sound/preamp the culprit?/external EQ needed?

If its not a big glob of gunk on the stylus then its probably the phono stage. No amount of EQ ever gonna fix muddy. Better phono stage. Night and day.

You can try moving your speakers out from the back wall it will lower the bass and open the sound stage 
The iPhone is not powerful enough to drive the amp 
 Rogue Audio make some good stuff I had a Cronus Mag and have been to the factory they are a great bunch of guys too 
Which preamp do you have 

Enjoy the Music 
Tom

I would also check the cartridge setup. First- how old is it and what sort of cartridge. If a moving magnet, its common to see the cantilever suspension perish after about 3-4 years. After this time the cartridge can take on either a stringent sound (suspension hardened) or a muddy sound (suspension is melting). It does not matter how long you've used it- it matters how old it is and what sort of environment its in- for example the suspension is more likely to melt if tobacco smoke is common in the environment.


If the cartridge is newer it might simply be how the cartridge is aligned in the arm, and you might also check the loading of the cartridge (if the preamp is so equipped).

You may also have a feedback issue. Turntables can be very sensitive to this and need to be well isolated. Kind of like having a microphone too close to the speakers on a PA system. However, with turntables, instead of a squeal, the feedback is in the bass region. 

Some have found that placing the turntable on a shelf extended from the wall provides a better platform than having the turntable sit on a cabinet or shelf assembly that sits on the floor.