so I swapped my straight through muffler back to the stock one and, I have noticed the difference in the low-end and high-end speeds. stock muffler = high low-end with lower high-ends & straight through muffler = low low-end and higher high-end speeds. Does anyone knows if this is true?

 

straight through muffler = less backpressure = loss in low end torque.

 

Small and restrictive exhausts maximize flow and torque production at low RPMs.

Unrestricted and larger pipes maximize flow and torque production at high RPMs.

Typically, a stock exhaust will produce more low end torque while aftermarket import exhaust systems focus on top end horsepower (a function of high RPM torque production.)

So, feeling more low end 'grunt' with a stock exhaust is normal compaired to a junky aftermarket/exhaust shop setup.