You asked how to get a flatter swing. I suggest to do this first in your shoulders rotational plane, and not forcing the arms to be pulled low into the chest.

Adjusting the shoulder plane will allow the swing to replicate this position vs. just a change in the arm's position.

To help you describe the change, imagine the shoulders rotating perfectly horizontal (or flat). To get your arms to swing about your body, they will have to be very vertical in respect to the body (what you want to get away from).

So to flatten the arm's position in respect to the body, if you tilt the shoulders rotation about the spine and get them to rotate closer to a 45 degree angle, the arms can now swing about the body on a flatter "looking" plane, yet they are actually now more perpendicular in respect to the body...which get's you to stay "connected" better during the body's rotation.

To answer your question about your swing going to a flatter swing plane...this is never a bad thing. Flat is better at connection timing. An upright plane is actually a lazy disconnection of the arms from the body that requires more timing. You may feel more powerful, but what is preceived as powerful is sometimes wasted when you misstime all those additional elements.

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I'm a golfaholic, no question about that. Counseling wouldn't help me. They'd have to put me in prison, and then I'd talk the warden into building a hole or two and teach him how to play. ~Lee Trevino