Connect quad l-subwoofer to vintage sansui 8 amp

First off, you will have to excuse my ignorance :-) I appreciate good
audio, but admit I'm a little lost on some of the more technical aspects.

Currently I am running a vintage Sansui Eight integrated amplifier with
two Duntech/Dunlavy PCL-15s. I love the sound produced by this system but
decided it needed a bit more oomph in the lower end, so I purchased a Quad
L-Series subwoofer.

I have seen some good reviews about the Quad, and it has rca line in/out.
I thought this would work well with the old amp given that it doesn't have
a subwoofer output.

Anyways, I plugged the LR “pre-out” from the Sansui into the LR “line-in”
on the subwoofer. I then plugged the LR “line out” on the subwoofer into
the LR “main in” on the Sansui (All RCA connections). I got the subwoofer
up and running but have noticed a decline in the audio quality through the
Duntech's. It seems that the sound has lost some of it's sharpness, and
changes in notes do not seem as distinct.

I have played with all the filters on the subwoofer, but I could not seem
to improve the sound quality. The quality still does not improve when the
subwoofer/filters are switched off.

I am just wondering if anyone has encountered a similar problem? Or might
be able to offer some advice?