Using Adcom GFA-555s as monoblocks to power magnepan 3.3s in active biamp

Bridging an amplifier effectively doubles its output impedance, which halves the damping factor.  A 4 ohm speaker remains 4 ohms, but the amp's ability to stably drive the load is compromised significantly.  So, you can safely bridge into 4 ohm speakers if the manufacturer has designed the amp to be able to handle 2 ohm speakers when used normally.

There's no correlation between a speakers output impedance & its sensitivity.  However, most SS amps produce a lot more watts into 4 ohms than 8, so bridging tends to be more attractive to people with 8 ohm or higher speakers.  The amps are more stable bridged into these loads.  One thing to note, though - reducing the damping lowers the amp's control of the speaker.  This could result in looser sounding bass.

To Tackle your simpler question:

Take a look at the Stereophile measurements on any speaker.  Regardless of what the manufacturer specifies as its nominal impedance, the solid line in the 1st graph will show the impedance (up and down) varies significantly by frequency (bass to treble is left to right).  The discontinuities normally correspond to port tuning, crossover frequencies, cabinet resonances, etc.  While the nominal impedance (along with sensitivity) should give you some idea of their power requirements, it's not unusual for the speaker impedance to vary from half (or less) to a few times that value.  It is quite common for the lowest impedance to be reached in the bass, where power requirements are highest.

You won't find that graph for SS amps, as their output impedance is usually a fraction of an ohm and much less variable across the audio band.  However, any distortion or clipping graphs will normally climb slowly on the right side of the graph, then suddenly and dramatically turn north.  You don't want to push your amp into this region!

Depending on the nature of the mismatch, the result could be an amp or speaker fuse blown, or a speaker driver or an amp output section.  Your house & neighborhood are probably safe, though!