I was out of town traveling last week and haven't had time to post about my experience using the Mixpre-6 the first time. Unfortunately, I had a pretty big failure, which ties back to design limitations (in my view) of the Mixpre. I have been able to do testing and am fairly sure of the problem.
Cut to the chase: the problem is that if you use an SD card that doesn't work (presumably not fast enough, though in my case supposedly Class 10), you can get short dropouts on the recording when the card can't keep up. Pretty typical behavior for SD card recorders. The bigger issue is that the dropouts you get on the SD recording also occur on the line output and the headphone output. That is, it appears that the analog outputs -- line out and headphone out -- are somehow configured within the Mixpre to be post SD-card, such that every single dropout you get on your internal recording will be transmitted to the analog outputs. Thus, you cannot use an external recorder attached to the line output of the Mixpre as a back-up recorder -- any problems/dropouts you get on the internal recording due to a slow/corrupted/fragmented/failing SD card will occur on the Mixpre analog outputs and your back-up recording will be just as corrupt as your internal recording.
I hate to have to come out of the gate with negative impressions. I have owned the Sound Devices MP-2, 722, 702, and USBPre 2 and have always found Sound Devices to make rock solid gear. I have done quite a bit of testing now and have confirmed that dropouts occurring on the internal Mixpre recording lead to dropouts at the exact point on the line output and headphone output. I have only had this happen with one of my SD cards (a Sony branded card) and never with any other SD card, and I haven't had any dropouts except with this Sony card. So the dropouts on the analog outputs are confirmed, though it is only my guess that the original dropouts are occurring due to a slow SD card. Sure seems like it though.
I'm pretty bummed about this. I sold my USBPre 2 to fund the Mixpre-6. My USBPre 2 was rock solid as was the Grace Design Lunatec V3 I had before it. With both of those preamps, I could use the multiple outputs available to record to 2 devices, such that I had a backup recorder at all times. (Really, I've been using backup recorders or backup recording systems going back to the unreliable portable DAT days.) In these cases, the mic preamp could be a common point of failure such that having 2 recorders doesn't really help. But the USBPre 2 and Lunatec V3 were so rock solid that as long as my batteries were good, the preamps were never a concern.
Conversely, I think SD cards are notoriously unreliable. Forgetting finding a fast enough and reliable enough card to begin with, even a high-speed, high-quality card can fail at some point or can get corrupted or fragmented such that it has a blip and you get a dropout. With the Mixpre-6 designed as it is, you cannot simply use a second recorder attached to the line output as a backup. You will need to have a completely separate recording system. I guess for my purposes, instead of using a Sony M10 as a backup recorder to the Mixpre-6, I will use my Zoom H4n Pro to record my second set of mics and will send the output of the H4n to the Mixpre-6 in case I want to mix those mics with my primary pair on the Mixpre. Unfortunately, in order to have a back up for the rare time of recording failure, I will now be using the more noisy Zoom H4n preamps rather than the much cleaner Kashmir preamps of the Mixpre-6.
BTW, if you've read this far, this happened with my maiden voyage of the Mixpre-6. I had just gotten the UPS delivery of the Mixpre-6 moments before I went to record Trey Anastasio Band. I took enough time to insure that my batteries would power the MP-6, but then took off for the show with no time for additional testing or understanding how to run the MP-6. I was able to set it up no problem, and ran my Sony M10 as a backup recorder just in case. For the opening band (Vulfpeck) I ran a no-name SD card and everything was fine. For Trey, I used a Sony-branded SD card which led to the dropouts I mentioned on both the internal SD recording and the external M10 recording. I then tested at home to confirm.
Ultimately for my purposes, I think the Mixpre-6 will be a great piece of gear and will probably almost always provide great results, in large part since I think it is an excellent sounding piece of gear. But unless I run some kind of backup using my Zoom H4n, I probably will never trust that I got a recording until I get home and can verify. This is something I never worried about with my USBPre 2 or 722, which does bum me out quite a bit.