If the signal is too weak I'm afraid that no re-patching will help.
Probably you don't have enough gain.
The HA400 is expecting a line level signal.

A Y connector from the RC-2 to the HA400 would give you both ears at least

any one worth their salt at Radio Shack should be able to help you with a solution to this problem.

and I believe you would want to put a stereo to mono adaptor at the end of your chain (right before your
headphones)

and a little word about investment: You get out of equipment what you put into it.

They always say in carpentry that you should always used the right tool for the right job.
This is so true in electronics as well.

They are making really excellent and inexpensive mixers these days (and they all have a powered headphone jack in them).
Yamaha and Berhinger both have excellent little two mic/4 stereo line input mixers that are around $100.

With them, you won't need that pesky impedance adaptor any more for your SM57 and you'll be able to start to mix
other signals into your mixer to send to your looper pedal.

YOu'd be amazed what $9 worth of parts (piezo buzzer, some stereo cable and a quarter inch jack) can do to make
almost anything percussive sound incredible. A little mixer is going to be absolutely essential if you ever are to
get sophisticated about your percussion looping setup.

Start saving your dough now for such a set up. YOu'll be so glad you did.

Remember, a looper and a mixer are every bit as much of an instrument as your hand percussion and hand drums are.
Don't scrimp too much on them.