pass labs xa30 to valve amplifier.

Pubul57, as best I can make out, Bobby is pretty conservative with that 89db rating on the Merlins.

Slam and attack with clarity? Tube amps can do that in spades. A lot of that has to do with the relationship between the amp and the speaker:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

Tube failure should not be a cause for speaker damage. In fact a transistor failure is far more likely to damage a speaker, which is why most transistor amps have speaker protection circuits, something not really needed in a tube amp.

James123, a preamp that supports the balanced standard is my suggestion. The ARC is one that does not. The balanced standard (what created balanced line operation) is supposed to make the system immune to the quality/length of the cable. If the preamp does not support the standard, you will find that the cables still make a difference.

Ideally to do this the preamp will ignore the ground connection of the cable.

If I'm not mistaken, the ARC may have troubles driving the input impedance of the Pass. That has been discussed in other threads.

As a general rule of thumb, its a good idea to allocate about the same funds for the preamp as you do the amp, but individual circumstances often create wide variations :)

No, you just pick the right position on the volume control and it works fine. The MP-3 has a custom-built stepped volume control composed of 96 resistors. It sounds as good as a bypass.