I’m not familiar with the internal design of the amps that have been mentioned, but I would expect that one significant difference between a bridgeable stereo amp and an amp that is internally bridged and is mono only is that in the former case two signal paths that are almost entirely separate would exist when the amp is operated in bridged mode, while that would not necessarily be the case in the latter situation, where the output stages may be mainly what is separate. Which means that it may be less challenging in the latter case to obtain an optimal match between the characteristics of the signal provided to the amp’s + output terminal and the signal provided to the amp’s – output terminal. And similarly that may be less of a challenge in the case of a “fully balanced” amplifier, which like a bridged amp provides a full amplitude signal on both its – and + output terminals. Fully balanced amps are often (although not always) designed with signal paths comprised of differential stages, and that approach may very conceivably also mitigate the challenge of keeping the two outputs matched, compared to situations in which the + and – output terminals are driven via separate signal paths.
In any event, I think the others who responded have hit upon the key points. As is usual in audio there are a multitude of approaches that can be chosen to meet a given set of requirements, each involving innumerable tradeoffs. And a designer will have his or her own preferences among those approaches based on the particular individual’s background, knowledge, experience, and the approach he or she feels most familiar and comfortable with. And what usually counts most is how well whatever approach is chosen is implemented.
Also, as alluded to earlier, bridging tends to have negative connotations in some quarters as a result of the fact that stereo amps which can be bridged usually sound worse when bridged than when operated in stereo mode. The main reason being that in bridged mode such an amp will see a load impedance equal to the speaker impedance divided by two. But I would not extrapolate from that any expectation of the sonics that would be provided by a monoblock amp that is internally bridged, and which can only be operated in that manner, and which has been robustly designed with the need to drive low impedances presumably front and center in the mind of the designer rather than being some approximation of an afterthought.
BTW, the reason a bridged amp sees half the speaker’s impedance is that from the perspective of each of the two bridged outputs a given output voltage will result in twice as much current being drawn from it as would be drawn from that output if it were operating by itself. If operating by itself the voltage appearing across the load would be the difference between the voltage it is outputting and the amp’s circuit ground, which the circuitry sees as zero volts. In a bridged configuration the voltage appearing across the load would be twice that amount, since the other side of the load is being driven with an equal and opposite voltage rather than being connected to ground. Twice the voltage across a given load impedance means twice the current. And since per Ohm’s Law resistance equals voltage divided by current, if a given output voltage results in twice as much current being drawn it appears from the perspective of whatever is providing that voltage that the load impedance has been halved.
Best regards,
--Al
