Answering your A question requires asking some other questions:
1. Have you run Audyssey?
After doing that, 0dB is "reference", which would produce the same volume levels as you should hear in a calibrated commercial theater. Many people find that to be much too loud in a home environment.
The Audyssey instructions in the receiver's manual are overly simplified.
Be sure to review the Audyssey 101/FAQ at http://www.avsforum.com/t/795421/official-audyssey-thread-faq-in-post-1/51750#post_21782993
2. Do you have Dynamic EQ enabled?
When you listen at volume levels below reference, DynEQ increases the sound levels of the lowest and highest frequencies to compensate for the roll-off in our hearing's sensitivity at those frequencies when lower sound levels are involved. See the Audyssey FAQ mentioned above for details.
3. Did you remember to set all of your speakers to be "Small"?
That enables bass management so that the lowest frequencies get redirected to the subwoofer. Your main speakers might not be able to produce the lowest frequencies.
4. If you've run Audyssey, did you verify that your subwoofer's "trim level" is not -12dB?
A value of -12dB means that the receiver was unable to calibrate the subwoofer. It turned down its own subwoofer output as far as it could and it was still too loud. You'll have to turn down the subwoofer's own volume control and then recalibrate. Again, see the FAQ for details.
B: With a calibrated system (i.e. after you've run Audyssey), using a more powerful power amp only increases the maximum possible undistorted sound level. It doesn't change the sound level produced at a given setting of the volume control knob. A large external amp is most useful with inefficient speakers and/or a large room.
Using the preamp outputs does not affect the ability to listen to discs which have soundtracks encoded with DTS-MA. The audio decoding is done digitally before the signal is sent to the analog preamp circuits. It can even be done in the BD player before sending the signal to the receiver.