This is what I posted on AudioAfficiando:
My initial thoughts on the MX-151...
As posted above, the MX-151 that I've been evaluating in home for a week will join my set-up permanently, replacing a Marantz AV8801 processor which I've had in place for about 5 months. Prior processors/receivers include the Integra 80.3, the Classé SSP-800, Denon 4311, and Denon AVP-A1 (non-3D model).
My amps are MC-302's and an MC-303, and speakers are 802D2 mains, HTM2D2 center, 803D2 surrounds and rears, and two JL Audio 113 subs.
I actually hemmed and hawed about bringing the 151 in, as I was really enjoying the 8801 (11.2 configuration with Audyssey Pro calibration), and had worked quite a bit to get the room configured for the expanded channels. Nonetheless, when the opportunity comes to try a 151 to audition in-home, that's tough to pass up. So in it came, and over the next few days, I reconfigured my amp and speaker wiring for a 7.1 hookup (slaving my 2 JL Audio subs off the 151 sub out channel), and leaving my 805s in the rear from the 8801 configuration for the moment (as I was not sure I was committing to the 151, I didn't want to move the 803's - used as wides with the 8801 - until I had a better sense as to what I was doing).
My initial listening was without a Room Perfect calibration, just set the levels and distances manually, and watched some TV and sports. My early impressions piqued my interest - very smooth and articulate, against a very black background. Still, no critical listening, and no Room Perfect.
Then I performed an Room Perfect calibration, achieving a 100% Room Knowledge with 17% correction. I started to play some Blu Ray's off my Oppo, as well as 1:1 rips off my Mede8er media server, and that's when I started to smile - a lot - to the point of shaking my head in acknowledgement of the quality I was experiencing.
I started with the 2013 DTS Demo Blu Ray, which has a lot of different cuts of movies and multi-channel music (it's not a perfectly authored disc however, as the levels aren't well matched between samples, and their promo clip interspersed throughout the cuts as no audio!) Still, cuts from The Hunger Games, Prometheus, Battleship, Safe House, and several other titles were simple compelling beyond expectation. Even though I dropped wides and heights, the 151 did such a great job with stereo pairing of all speakers, the envelopment was wonderful. The first music-only cut, "Listing" from Minus the Bear (a 7.1 encode) was so unreal (well, meaning REAL) that I could SWEAR there was sound coming precisely from my disconnected wide and height speakers, which was impossible - but I literally got up and walked over the speakers, finally breaking the imaging and proving I was not insane. The imaging between all adjacent speakers was that good. A friend and fellow hobbyist came by, and when I played the cut, he felt the same exact way - we were staring at speakers that were not connected (heights and wides), believing sound was emanating from them, While this disc (and title) was great on the 8801, I did not get the same surreal effect when listing in 7 channel mode with the Marantz.
I also played Monsters Inc., Rush: Time Machine; Porcupine Tree: Arriving Somewhere (an excellent DVD audio track); Dave Matthews/Tim Reynolds: Radio City; Chris Botti: Live in Boston; Avatar; and other Blu Ray titles. All awesome.
The 151 produces a very dynamic and unencumbered sound, and Room Perfect is so different than Audyssey it's remarkable it's not more well known or licensed (I guess the cost is quite high). Certainly, low frequency performance, even with slaved subs vs the independent subs on the 8801, was by far the most noticeable improvement - so much more articulate and clear, and my concerns that I would miss the lower volume benefits of Dynamic EQ were vanquished - bass and LFE performance is properly engaging at all levels. Listening to various concert material particularly revealed this - the sound of the kick, snares, and toms was more visceral than I've heard before.
But midrange and higher frequencies seem more open too, or perhaps, less processed - whatever it is, I can make an analogy to digital photography - Audyssey Pro, in various processor embodiments, seems to provide really wonderful quality, but perhaps mildly compressed jpg images. The 151 appears to deliver uncompressed RAW files, and Room Perfect is optimizing within this expanded space. And the sound is crisp and clear without ever being harsh - not (again, using the digital imaging analogy) because of edge enhancement, but because of employing the audio equivalent of a high quality lens and large, ultra-low noise sensor.
Maybe this comparison is not fair to Audyssey, and perhaps the MX-121 sounds appreciably different than the 8801. But I can only comment on the various Pro calibrations I've done on my past processors, and while there are differences amongst them, the core impression is the same in comparison to Room Perfect.
This weekend, I moved some furniture around and did another RP calibration, 13 point / 100% Room Knowledge with 10% correction - will be doing some more attentive listening over the next few days, though there was some ambient noise in a few positions so I may re-do (BTW - is there a way to cancel the current room position and re-do, rather than aborting the whole process? If not, that would be nice to have added via firmware - with Audyssey Pro, you can do this if, say, someone decides to drop a bowling ball in the room above during your ninth position).
The 151 has some room for improvement, and it would be interesting to see what a 160/1 may bring in the future. I also wish I could understand (if possible) how to connect two subs independently and not have to configure my mains as XL or XXL, implying they would get the LFE signal while the subs duplicated this material (this is sort of what I understand to this point from McIntosh, though I may be getting this wrong. Still, its not as easy to set up independent subs as it is on the 8801, 80.3 or SSP-800). Yet the sub performance is so nice and impactful right now that it's more of a curiosity vs a necessity. I also find Surround Modes a bit confusing (I can have a display read-out that says: Proc: (Games) / DLPIIx Music...what does that mean?) It would also be nice to be able to keep the processing mode on the display, but it always reverts back to the input screen.
Overall, this processor does lots of things so well, I'm not bickering much. I'm not overly concerned about 4k, as I use an external Lumagen, and would probably get their 4k unit if and when necessary for my VPL-VW1000 - but it seems content is so far away, its not a very pressing issue.
Yes, the 151 is pricy, and perhaps future iterations will come in at different price points. But unless they find something even better, I certainly hope Room Perfect remains in the McIntosh lineup as new generation processors are designed.
As a side note, I use Roomie Remote, and I purchased the iTach serial device and programmed the serial codes to control the 151 without a hitch. Roomie does not yet appear to support feedback from custom devices, so that's a slight bummer, though not the 151's fault, as there are ample status codes available. The 151 also takes a while to boot/initialize, so this can complicate programming activities if you want to send a string of commands to the 151 (programming delays is possible in Roomie, but since you don't need them once booted, it slows down activity switching) I may just create a power up activity, and leave it at that.
The Mcintosh 151 web interface is also nice, and complete enough that I don't need to go into the on screen menu.
A bit wordy I guess, but hopefully this helps address some of your questions..