Sorry about the mistake with the Cary. I read a review of the 303/200, and it seemed to have a volume control.
In this case, I would prefer using the parallel speaker wire to the sub. If you use a splitter placed at the output of the Cary (or the input of the Avatar), you have no control over the volume presented to the sub. With unity gain, the sub is likely to be too loud, out of balance with the volume of the Sonus Fabers. With a parallel speaker wire configuration, the Avatar's volume control would work for both the speakers and the sub. (The sub should have a volume control intended to match the sub's volume with the main speakers' volume for first time setup use.)
With respect to the crossover, many subs have a crossover control, so that the sub will operate at frequencies where the main speakers are unable to perform. If your Grand Piano's give up below, say, 30 Herz, then the sub should be set to operate below 30 Herz only. In practice, speaker and sub frequency responses tend to slope off, not start or stop in cliffs at specific frequencies, and there will be an overlap, so you might want to adjust the setting so the overall effect of the combined speakers and sub is smooth.
Some subs have a phase control, too. This is used to manage any cancellation or reinforcement of the speakers and the sub where they overlap. For example, if the sub is out of phase with the speakers, it will tend to cancel out the speakers' output for the frequencies where they overlap. If they are in phase, there will be reinforcement. Personally, I prefer to set the phase so there is reinforcement, then I lower the sub's crossover frequency to reduce the overlap until the output is as smooth as possible. Good luck.
