There was a three year time span between when I sold my MC12BEQ and purchased my LS10. So I really can't offer a fair comparison. However, I will offer my thoughts.
My MC12BEQ was the non-HD version. I used analog outs from my Oppo BDP103 into the MC12 so that I could take advantage of DolbyTruHD and DTSHDMA. That being said, I preferred the sound of vanilla DTS and Dolby Digital using a toslink digital optical cable. That was mainly due to the superior DAC's in the Lexicon.
What I also loved about the Lexicon was the room EQ. I enjoyed how it corrected the lower frequencies without affecting the upper frequency range.
In summary I loved the MC12. The reason I sold it was I was starting to have issues requiring repair and I decided it may be best to sell while it still had value. Also, I wanted to move up into the digital world with HDMI switching.
Due to all the hype at the time I purchased a Marantz 8801 with Audyssey XT32. As good as the Marantz was, I always felt it was lacking in sound quality compared to the MC12. What the Marantz did offer was a decent DRC with Audyssey XT32 and the added convenience of HDMI switching. I also liked the ease of setup compared to the MC12; though that was not important due to once its setup I don't necessarily change things often.
The Marantz also never sounded as good to me with music as the MC12. For the three years I owned the Marantz I missed the Lexicon sound and was continually researching for a new SSP that would bring back the smile on my face the Lexicon provided.
Fast forward to now- I took a leap of faith based on comments here on the forum, the fact that Dirac was an option, and the reviews claiming that sonically the LS10 sounds nearly identical to the RS20i, I purchased an LS10. I didn't need the additional channels or the additional capabilities offered by the RS20i.
I will skip giving my impression as I did that a few posts below. However, to answer your question, the LS10 with Dirac outperforms the Lexicon. I feel satisfied (for now) and have the same sense of contentment I had when I owned the Lexicon. My love for music listening has returned with the LS10. I still get goosebumps when I listen to Tidal postprocessed with DTS NeoX multi-channel. This sounds better to me than Lexicon musiclogic7 did.
You're in a tougher position to decide than I was since you have the HD version of the MC12. I once considered buying another MC12 and adding a minidsp ddrc88a, but ultimately decided against it due to the addition of another D/A conversion and all of the cables required to add another box. I also decided I wanted to buy new and have the ability to be Atmos ready when I get the time to re-build my theater.
Something else I've come to appreciate, that the Lexicon cannot provide, is the ability to set-up and control my unit with my ipad. I can make changes on the fly during a movie without anybody even knowing.
Good luck with your decision!
Current Equipment: Datasat LS10 w/ Atmos and DIRAC. ATI 6005, AT527NC, Aerial Acoustics LR5's (LCR), SR3's sides, LR3's (rears), Seaton Submersive HP, Marantz VP15s1, 123" diag 16:9 Stewart Cima Neve screen, Oppo BDP-103, Palliser Flicks Theater Seating AC Power: Eaton whole-house surge protector at main panel, three dedicated 20 amp circuits, Surgex XR315 surge protector at equipment rack, Cyberpower 1400VA/900 watt, true sine wave UPS.
Last edited by jdlynch; 08-25-2016 at 06:37 PM.