I wrote the review of the 840W (as well as 840C player and 840E preamp) for Dagogo.com.
I would caution you in regard to the suggestion to add a power conditioner; this will take away definition/microdynamics typically as it emphasizes macrodynamics. If a person cannot hear such detail well they will think the definition has increased, when in fact it has not (Elizabeth, I am not targeting you on this, though I know you have some hearing loss. I have arrived at this conclusion about how people hear the effects of conditioning independently). If your hearing is good, you will note a loss of absolute refinement with addition of a power conditioner. Note also that I listen in a room which is custom for listening and has approx. 8dB lower noise floor than a typical quiet home room, so I can hear a tremendous amount of definition/detail in recordings. In some environments it might be difficult to hear such differences.
I do not use power conditioning on my systems for the reason stated above, though i have tried several. I consistently note a trade off with use of power conditioning versus an absolute gain. Even power bars can be damaging to the sound. You will note a significant change with the use of a power conditioner, but the addition of an extra component will NOT yield a more clean/pristine signal. Whether you prefer the sound of a single amp with power conditioning to two of them in Mono without is your choice. YMMV and I am not interested in debating this point. Feel free to disagree, but that is my position. If at some point in the future my viewpoint is changed through use of power treatment components then I will recant my conclusion, but until then I'll stand with my findings.
It is absolutely wrong to suggest that using the amps in Mono mode will not give more detail and focus. You will obtain a lovely improvement in those areas, as well as the obvious dynamic and headroom improvement due to the increase in power. The result has been the same with every amp switchable to Mono; improvement holistically. This is not to say that a more powerful stereo amp cannot outperform, but in my experience switchable amps are better when run Mono than in their stereo mode typically. It might cost appreciably more to secure a superior sound from a stereo amp vs. using two 840W's in Mono mode.
I appreciate Lcherpkai's input, as it is accurate.
Lcherepkai, I would suggest that the soundstage does not diminish with the Mono mode of the two Classic One MkIIII integrateds, but that it is filled in back to front more completely. This has the effect of having more forward presence and can seem like it's not as deep. However, I have not sensed that the soundstage has shrunk as much as it has been filled forward. On many recordings I hear more ambient information of the recording venue with the Mono mode than in stereo. To me that says the soundstage certianly has not shrunk. Again, ymmv.
I do strongly suggest you try several power cords, as these also can make a critical difference in the sound of the amp. In fact, the selection of a poor power cords can kill a system, while selection of a good one can make the performance shine.
Finally, watch carefully IF you are using very difficult speakers to drive, such as full range ESL with a low impedance. The 840W went into protect mode when I tried to run it Mono into the King ESL (2 Ohm). But it was instantly revived due to the superior safety electronics and never did so with any 4 or 8 Ohm speaker. I used the amps in Dual Mono mode for the King ESL often without any incident.
Again, I'm not out to assault anyone here, just sharing my experiences over the years with stereo/mono amp mode. :)