L Pad Control ruin sound quality???

What you were told is essentially correct. Although it is always possible that a solution which is technically non-optimal may happen to be subjectively acceptable, or even preferable, by compensating for other issues. But I wouldn't recommend going that way.

Also, depending on the power levels that are involved you may have a problem finding a good quality L-pad having an adequate power rating.

I'll mention, though, that I suspect the statement that "the more I reduce the volume the greater the negative effect on speaker control" is a bit of an oversimplification. I haven't taken the time to analyze that in detail, but it may be that the worst case would occur when the two variable resistances in the L-pad are set somewhere around the middle of their resistance ranges, the exact point depending on the characteristics of the low pass filter which you appeared to indicate would be "ahead" of the L-pad.

My suggestion would be to insert a volume control function between the preamp and the amplifier you are using to power the sub, implementing it with either a passive device or an active device depending on the impedances and cable lengths that are involved.

Regards,
-- Al

L-pads are usually only used for tweeters because of the power handling.

For subs, look at Goldpoint attenuators or the Emotiva passive control. Another option is a pro amp with gain control. Even better, NHT X2, Paradigm X30 or Reckhorn.