Wonder where he placed them, that had an effect one the way the sound is tuned. Often if you place them further from the axles you will get more sound reduction, the closer you get you will loose less usually. You can see this with were the stock V6 places their resonators before the X as opposed to the GTs where they are placed after and almost middle of the mid pipe. Less distance from the mufflers.
From my understanding the length from where the wavelengths are slowed down, to where they exit has an effect on the sound output. But I also suppose that muffler design may also have a lot to do with it, if they are chambered no packing like MAC flowpaths or like Dynomax strait throughs with louvered cores and packing material you will get different results with resonators.
Chambered mufflers may get greater results with resonators over strait throughs. I do not know where the roush ones fall. never dissected those.