With the set up you have(incl the headers which IMO are a useless mod-but will
give you a more wide open sound)you are gonna get rasp or what I call,
exhaust overide.I'd think your options to reduce this is to replace the resonator
with one that offers a more silent tone.It will also give a greater exhaust resistance
that would make me wonder why did you select the components you bought.
They were gonna make a louder exhaust.The HFC'S make it louder.
Another option I have used that works is to install tailpipe exhaust tip resonators.
These are small mushroom shaped pieces connected to a short tube that install
into the exhaust tips.Don't know the inside diameter of the Stillen system.There are
resonators available in different diameters.The only downside is you may have to drill
a hole in the onderside of the exhaust tips to hold the resonators in with a screw.If interested,check online for universal fit resonators.