I have no problem with the SD products I have used. I have never run schoeps, so I can not cast opinion there. I have run other microphones such as Sennheiser, Neumann, Miktek, DPA, Shure and Audix through a 552 & a MixPre-D with great results. I have not recorded on any SD 7XX series recorders, as I use other recorders for my use.
Maybe it's the Mic's ?
Running them with the Neumann's, Telefunken's and Miktek's have been extremely warm and full for me.
Can't speak for the others. It's all good. The 688 goes way beyond what most would need or use here, but t is always interesting to read up on new product-technology and drool a bit at new options. Would be real cool if it had Bluetooth capability! What's next to hit the market? Pretty interesting past years with what's been hitting the market. I'm looking forward to what's coming up next in this market.