I am using a similar set up with my Icom 746.I use
a Behrenger mixer and a Rigblaster Nomic in
just the way you descibe with no problems.
I feed the rear panel ACC jack with the mixer
to drive the balanced modulator directly. So far
no problems with RF or feedback. I did take the
extra precautions of using double shielded "data"
type cables, audio isolation transformers,
and ferrite chokes on all cables...so your
mileage may vary.
Also, you might be interested in posing this
question on our new ssb audio discussion
You can join by going to:http://groups.yahoo.com/group/ssbaudio/
Les Rayburn, N1LF