2.0 to 2.1 - will this work?

In order to add more bass for DVD recordings I have decided to add a subwoofer. Currently looking at lower end B&Ws, PSBs and NHTs.
My stereo system consists of:
Triangle Titus XS bookshelf’s
McCormack DNA 0.5 (100 wpc) Amplifier
McCormack Micro Line Drive Preamp
Preamp has both an active and passive ‘line out’ sets.

For a powered subwoofer with line level inputs, would it be advisable to ‘line out’ to subwoofer with one set (active?) and to the power amp with the other (passive?), and not use the speaker binding post on the sub? Main speakers in this setup would remain attached to the McCormack Power Amp. My logic in favouring this setup is to keep the higher frequencies from being filtered through an additional amp (that of the sub).

Your speakers will be happier not seeing bass frequencies, i.e., you should want the higher frequencies filtered. So consider a sub with bass management or an external BMC.

Depending on the size of your room, the SVS SB12-Plus would be a good choice.