My take on the Flowmasters is this (talking about the American Thunder axle back) is that they have a nice sound at lower rpms, but they tend to have a tin-like sound and some rasp at the mid upper and higher rpms. Check out the videos of them on the first page to get an idea. Here is another vid of them as well:
I'm not really sure what to recommend to you to get a deeper sound without being much louder.
You may want to look into cutting off the stock resonators (not the cats) as many people say this tends to give a deeper more clear sound. It will likely also make it a bit louder, and may add some in-cabin drone but that seems to depend on the exhaust the car is running. It would be something cheap to try thats for sure. Just keep them once you have them cut in case you decide to add them back in later.