Depends on the quality of the amp. Most amps when bridged tend to see a rise in THD, where as a dedicated mono will not.
This totally depends on the design, IMO a "bridged" amplifier sounds superior to a single channel amplifier.
If the amplifier is sturdy enough to handle low impedances bridging it is a non issue. The only issue there is with "bridging" is that the amplifier sees a halved impedance, is if your speakers are 4 ohms the amp sees a 2 Ohm load.
There are many advantages to bridging a amplifier, for once you get 4 times the power, this because the speaker is now connected to two identical amplifies one end of the voice coil on a positive channel the other end on a negative channel, which is either driven by the negative side of the differential input on the positive channel or by a separate preamplifier channel. i.e. the negative node in a balanced signal, the preferred way.