NC 700 Echoing When Monitoring Live Audio
I've been using my NC700s for some months now and have come across an issue that I haven't been able to find anywhere else online. I had originally tried to use the headphones wired to monitor a microphone (Shure SM58) when recording into a mixer (Yamaha MG10XU) and noticed that when talking into the microphone my live voice feedback into the NC700 would sound like talking into a tin can, the audio quality in the headphones was very echoed, tinny and completely off-putting. I decided to switch out the headphones for my QC35 to test, and the problem disappeared immediately - the audio sounded perfect. As soon as I plug the NC700 back in, the tinny, echoey mess returned. I thought, surely it must be the mixer. I switched the mixer out for my Shure X2u audio interface, plugged in the same microphone, NC700 into the 3.5mm port and same thing again. I tried one last microphone, a Blue Yeti Pro with its own inbuilt 3.5mm audio output on the bottom and again, the same problem. I immediately assumed at first that it would have been an option such as "self-voice" in the headset kicking in or something but it wasn't as, that was one of the first things I tried to turn off, after doing so the issue persisted. I originally spoke to Bose support about this who went through a whole load of other basic troubleshooting to no avail, and ended up having the headphones replaced for another unit. After the replacement arrived, same thing again. It seems as though somehow the NC700 have a flaw where they appear virtually useless for any type of monitoring scenario. Support told me "strictly speaking the Noise Cancelling 700 are not advertised as being suitable for audio monitoring purposes." I found that absolutely ludicrous. The model beforehand is perfectly capable of this basic function as well as just about every other pair of wired-capable headphones - and - there's even a series of YouTube videos on Bose's channel showing various predecessor headphones being used for this very purpose! Has anybody experienced this or been able to replicate this using any sort of microphone or audio interface where they can listen live to their own voice through a microphone?
TL:DR: Tinny, unusable audio when using the NC700 to live monitor a microphone.