Would be very interested in hearing your opinion. I am pondering doing it, but when you’re in the $150k range at today’s prices there’s a lot of options. If you did it, please share your thoughts. If you thought about it and went a different direction I am curious about that too, where did you land?
What I have today sounds amazing. I am running pretty efficient speakers, but I unreasonably itch for a bit more oomph.
Thanks!
Matt
I have the M500 mono-blocks. I can’t give you the direct comparison you probably would like to have, but there were a big step up both in sound and imaging from the McIntosh MC901s that I had, even though the M500s had less nominal power than the MC901s. Unfortunately, for comparison purposes, at the time I made the change my speakers changed too… but the old ones had about the same sensitivity as the new ones.
Before I had the MC901’s I had a McIntosh 250 watt per channel stereo amp. The change to the 901’s was an improvement but… that involved a speaker change too.
Amp’s specify RMS power into some impedance. I’ve always thought that was not a very useful spec’. I don’t think I have ever had speakers that ran at close to 30 watts RMS, even when they were turned on full blast, too loud to listen too.
To me, what counts is instantaneous power and the ability to drive low impediance.
Speakers companies don’t tell you how they measure their speaker impedance, but I’m pretty sure what they measure is a static impedance. What happens to speakers during transient pushes a speakers impedance to a lower number, for an instant, than they are spec’d for. IMHO.
I think that the M500’s made such a noticible improvement is because the M500s excel at dynamics and low impedance. Again, IMHO.
I think for imaging and transients instantaneous power and driving low impedance counts. Unfortunately what manufactures of speakers and amps give you isn’t very useful for making a determation, by the numbers.
So, ignoring the speaker changes, going from stereo to mono-blocks improved my system but going to the M500s was the big improvement.
When I got the M500 monos I considered the the M500 stereo, but my belt and suspender attitude drove me to the monos and I’m glad it did.
Dan
In any case, especially regarding current delivery at low impedance, when going for mono blocks it should be „real" mono blocks instead of two stereos in bridged mode, since bridging doubles the minimum impedance. (And it halves the damping factor, too, hence the ability to control the woofers).
@itoakio : the S500 and M500 are internally bridged amps. Both connections for the speaker cables are „hot". The user is told not to connect one of them to earth.
@Dan : from my personal experience with McIntosh with one amp ( 2 x 450 W per Channel ) and other amps of Musical Fidelity and Accuphase I assume that it is Not difficult to beat a McIntosh amp. For many listeners these amps are ok, as they deliver a lot of power, which is ok for hard rock and so on, but if you want to hear very quick transients and subtle nuances in classic music „your feet fell asleep" as wie say in Germany.
@matthewk I am in a similar situation as I own a S500 too. And at present I do not miss anything 
But as I have a pair of speakers which allow bi-wiring ( which I use ) or bi-amping I also think about buying a second S500 and making bi-amping? Which means I would have to buy only a second S500 instead of a pair of M500.
Before the S500 I owned two Accuphase A75 which are class A stereo amps but can be used for bi-amping or in bridged mode too ( just an adjustment at the front paneel ).
MSB does not offer the possibility to easily switch between stereo mode and bi-amping mode ( bridged mode is not possible as the S500 is internally bridged ).
Perhaps Daniel Francis may say something about bi-amping with two S500?
So it may depend on your speakers whether two S500 in bi-amping or two M500 in mono-amping may deliver the better result. But we should keep in mind that the input stage of the M500 is better that the input stage of the S500.
I can’t speak directly to your question, but I do have a thought to share. For many decades I wondered just how close I was to the limits of the various power amplifiers I owned. I bought a preamp 6 months ago that allowed me to configure the displayed volume in a way that makes it easy for me to mentally calculate the peak power that is possible from the power amp given a full scale signal from the DAC. The results were enlightening. I typically use less than 2 watts peak, so a fraction of a watt average. My more exuberant sessions use less than 20 watts peak. (For reference , my speakers are 87db efficient and my listening distance is 12 feet. The speakers have an impedance of 4 ohms nominal and 3.1 minimum.) I doubt many people with multi-hundred watt amplifiers are driving them anywhere close to maximum power, even for the transient peaks. However, the industry standard is to quote maximum power at 1% distortion. Class A/B amplifiers designed with minimal negative feedback typically have very low distortion at 1 watt and linear increases in distortion all the way up to 1% at rated power. So the reason to buy a big amplifier of this design is for the low distortion at 1-50 watts, as opposed to the big power ratting at 1% distortion. I would respectfully suggest you probably don’t need more oomph. That doesn’t mean the monoblocks wouldn’t sound different. I again can’t speak to that.
I went from 205’s to 500’s and can without hesitation say they have better control but I too upgraded the speakers at the same time so I can’t really give you an A/B comparison.
What I can say is the M500’s handle my speakers which have a fairly low impendence dip without the slightest issue at both low and high listening volumes.
I had no idea the stereo s500 was internally bridged. Interesting, perhaps that’s why we don’t see 2 ohm power rating in the technical specifications?
We offer a great upgrade program for these amps, we can bring your stereo amp in, recap it to match a new M500, and send back a complete set of M500’s 
Bi-amping with two S500’s would be awesome with the Sentinel DAC’s 4x output mode, but wouldn’t benefit otherwise with a Cascade. You would be throwing away half the amp. The M500 does have an improved input stage over the stereo model as well and would be the best possible performance.
That being said, the S500 alone is pretty damn good 