Integrating a subwoofer

the subwoofer (connected through loop out)

This is your problem. What you want is pre-out or sub out. Loop is fixed line level. Essentially a bypass, it gives no volume control over the sub. Because of this it is simply playing way too loud.

Your amp does not have pre or sub out. The solution is a $20 Line Out Converter. You might be able to find one at Parts Express for even less. Don't pay more, it does no good. Connect the LOC to your speaker terminals on the amp, then connect your RCA interconnects from the LOC to the sub. 

Thank you all for the advises and corrections. While waiting for the LOC to arrive, I will go over these suggestions before devoting to a DSP. Also, I replace the softer rubber feet with the anti-vibration isolation pads that was used for washer and it helps.

Again, thank you all for your inputs, especially the correction and suggestion of line out converter (LOC) from millercarbon. My sub finally comes alive and is even better than before with my Cambridge integrated amp when connecting to preamp out. Much of refinement now is also attributable to the HPF on my parasound, where I still set it at 20 hz. I found at 20 hz the bass from subwoofer is more weighty and still controlled. At 40 hz, the low end lacks weight/extension and not as dynamic. Also, the sweet spot of crossover on my subwoofer is between 60 hz and 70 hz. Beyond 70 hz, the mid-bass starts sounding a bit too much boosted. I was never aware Einstein is also an audiophile genius.