A bit late, but as promised, here are pics of inside the RNDI-S for anyone curious.
While I had it apart, I also took the liberty of rearranging the shell and internal brackets so the front panel features the flat edges of the shell instead of the default slanted edges (personal taste).
Behaves very predictably as I had hoped. Everything I print through it opens up very nicely. With the MixPre's routing matrix, I also ran a mic into channel 3, channel 3 to L&R mix > L&R mix to Stereo Out > Stereo Out to the RNDI-S, then back into Channels 1-2 (making for a signal chain of mic > preamp > RNDI-S > preamp. Compared to the original signal, the experimental chain sounded better, as if the extra stages "fixed" something. Hard to place my finger on it, but it just sounded more natural through the longer chain. Don't know if anyone can elaborate on why the would be, other than the benefits of a longer analog chain serving to smooth things out a little. It had the same focus, just more natural in terms of what I was perceiving frequency-wise. I imagine splitting the mono signal into L&R and processed through separate transformers and preamps influences that in some way as well. FWIW, the mic was a TLM 193, which is flat and natural already. But the chain sort of sounded like some very subtle and natural EQ had been applied. Don't know if that's more of a testament to the mic or the preamps having a strange character that wasn't noticeable until the longer chain seemed to "fix" it. I suppose I could try to measure that if anyone is curious enough. The only downside is larger file sizes, but storage is cheap nowadays and I'm not busy enough with audio for that to be a problem right now.
I'm also curious to see how the extra stages will effect bass guitar (Bass > RNDI-S ch 1 > MixPre preamp ch 1 > RNDI-S ch 2 > Mixpre preamp ch 2) compared to DI > MixPre.
I have no other outboard, so it feels like a nice hack for taking full advantage of the MixPre and RNDI-S together at all times on projects that warrant it where every little bit counts.