dual exhaust too loud

Has anyone out there had experience with dual exhaust on a 2000 sierra z71?
I just had a "performance" muffler with dual pipes installed on mine with 3" chrome extensions. I thought I'd enjoy the sound of duals and pick up some added power and fuel mileage.
Problem is.... at about 1700 rpm the sound reverberation in the cab is so loud, it is deafening. It doesn't happen when revving the engine in neutral, but does so under load and in any gear.
I took it back to the installer who installed a small resonator ahead of the main muffler. This didn't change anything as far as I could tell.
Can anyone give me an idea of how to correct this, short of installing a new stock muffler?
( Incidentally, I do like the additional power with the performance exhaust)

On my 99 Sierra Sportside Z71 I cut the Y pipe where it branches and add another pipe down. Removed the main muffler cause the pipes run side by side. Actually running off the converters only and has a great rumble. Did a decible check to see if I may have problems with the cops and only 68 db at idle 80 db at 2000 and 95 db at 4000 rpm. Just a little above Provincial guidelines at high RPM. It is a pretty sweet sound in the cab, not overly loud or uncomfortable. Neighbors haven't complained yet. But unbeliveable power and even crisper shifts.

Thanks for the response. I don't have any idea where I am with decibels, but I am running 2 converters into a Y and then into a "performance' muffler, which has 2 outlets and rub 2 separate pipes from there, exiting on the left and right hand corners of the bumper into 3"chrome pipes.
It sounds odd to me that you are able to run straight back from the convertors and not be too loud. Makes me think that my muffler is developing some sort of harmonic resonance (if there is such a term)
All I know is this combination is not working like I wanted. I like the sound on the low end and upper end, and I do like the added power and you are correct about the improved shifting.