Seriously, you simply cannot track your synths right without this RNDI box. At first I bought the mono version and then wanted also stereo to track my stereo synths. The sound of it is really exciting with any preamp. I tried no less than 5-7 different preamps. It works marvelous with each, even with the simplest (cheapest).
How does it sounds? It is FET active circuit with custom RND transformers. So it is prominent, noticeable, rich and 3D. It has this Neve influence. Having this sound captured right from the source, you can sculpt it later more flexible. If you know the struggle when you are trying all the processing possible but your line still doesn't fit... So this box designed to solve problems. I found myself using less EQ, compression and other processing because I'm getting right sound from the source.
Be aware that RNDI demands phantom of +48v. Box getting it from the preamps' input thru the same cable. And since you will be using it, you need to be aware to not to send your +48v preamp power somewhere else. So just be careful about it.
And the last about sound sources... You can connect literally any! I tried to send the signal from my phono preamp to RNDI to capture my vinyl in the balanced sound: it was amazing! Any line level source this box eats with no problem! As well as usual line level signal from synths (and everything hotter). So you can connect samplers, drum machines etc. I tried to track thru RNDI Akai Force: makes the digital sound sparkling and real! I've sold Force, but have no plans to sell any of RNDI. They are beautiful!