Even without calibrating, having the volume at 14 on a Dolby Digital/DTS source sounds very high to me. I did read somewhere that the dB voltage scale, or something like that, has 100 watt power-amp pre-amps having a maximum volume of something like 18.
You could check to see if there is a short-circuit occuring with the speaker cable.
I would have thought that a 100 watt amplifier would be sufficient in most situations. If your set up is in a large room and you are sitting some distance from your speakers, then you'd need probably a power amp of ~300 watts to get really high volumes, maybe even higher if your speakers are demanding.
A possible issue could be the LFE channel. If you're not using a subwoofer, and the amplifier is sending out the LFE to your main speakers, then this could be causing the overload. To disable the LFE channel, turn the subwoofer on in system settings, and set all your speakers to 'large'.
You should note that without a subwoofer, you are losing a large part of the possible dynamic range in DTS/Dolby Digital soundtracks. This could be why you are disappointed with the performance of your system. Use of bass and sub-bass (i.e. floor-shaking) is encouraged by THX as this is less tiring for audiences. To get true dynamic range from noisy DD/DTS soundtracks, your speakers and subwoofer have to be very capable of bass. This means 15 inch driver(s) on your subwoofer and big, efficient, floor standing speakers.
One other thing is that DTS soundtracks usually seem to have more bass than DD ones, so the DTS cut-off at a lower volume isn't at all surprising.
