Best Subwoofer Connection

OK, now I'm a bit confused. If I go from the preouts to the line in (and run the mains full), then I can use the external xover via the controls. If I go from the sub's line out to the amp in, does that mean I have no control over the xover, and it just is what it is internally? Sorry to ask somewhat noob questions, but I have had very little experience with subs.

I believe all equipment will worked best within their comfort zone, I recommend use the sub line-output high pass the signal to feed the power Amp, your Amp and speakers will thanks you for doing that. 

The sub has a crossover frequency knob that you will use to adjust the crossover point - that frequency at which all signals above it go to the power amp and all signals below go to the sub.  This is adjustible between 30Hz and 130Hz with 80Hz being a good starting point.  I would bet your setting will be much lower due to the good low freq response of those B&Ws and your taste for more bass.

The first option is to Run interconnects from the pre outs through the sub and back to the A300CR.  This uses the crossover in the sub and relieves the A300CR and B&W from low frequencies. You set the crossover to determine what the crossover freq will be.  Given what those B&Ws are capable of in low freq, you will have to adjust, listen, adjust, repeat.

The second option: Even if you decide to run a second output from the preamp and not use the hipass on the sub, running the amp and speakers full range,  you'll still have some crossover adjustment due to the overlap in frequency response between the B&Ws and the sub.   

@dbtom2  ... thanks, man! That's exactly what I wanted to know. I'm going to experiment ... now, at least I have a clue. Thanks, again.

Thanks to all who have commented and made suggestions. I now feel confident to proceed because of your assistance.