You may wanna check into hooker aerochamber mufflers, they actually flow real well, if not better as compared to other mufflers, not too expensive either.
If you wanna deepen your exhaust note, you should consider an H pipe. If you want to gain noticable power from idle to whereever over dual pipes alone, you may wanna consider an addition of an X pipe (I tried this, it quietened the tone big time, and brought back noticeable power)
Most people think glasspacks flow wonderfully due to the straight thru design, thats not entirely true, once you take into account the louvers inside and how they disrupt flow.
I added a few soundclips in the bench racing section, titled "more I-6 videos", should give you and idea of what to expect.
