Cav, the first point is that it's unclear how this would have the effect with two-channel SACDs that you describe. In any case, most sub manufacturers(including Axiom for the 350)caution against making a simultaneous speaker level and line level connection. Axiom doesn't go into detail, but some others specifically warn about damage to the sub and receiver amps. Some subs apparently have totally separate input circuitry for the high and low level inputs, which prevents any interaction, and state that such a connection is okay. At a minimum, contact Axiom for further details on their circuitry before you do this.
As to your second question, sub amps typically have input impedances on the order of tens of thousands of ohms on their high level inputs and, because of this, essentially zero power is transmitted, just the signal information, and full power is still available to the other speakers. Another result of the high impedance is that although the sub and speakers are connected in parallel, there's no significant lowering of the impedance seen by the receiver. The net impedance for parallel connections is such that its reciprocal is equal to the sum of the reciprocals of the impedances of the loads. So for example, with a 32,000 ohm sub amp and an 8 0hm speaker connected in parallel the total resulting impedance is 1/32,000 + 1/8 = 1/T; 4,001/32,000= 1/T ; T=7.998 ohms.
