I have the good fortune of owning two System Ones. One standard system and one rare "high-power" configuration using 2 620 amps and the uber-rare BA-100 bridging adaptor. These 2 systems are both near mint and truly the cornerstones of my vintage collection.
I also owned the hi power system when it came out & the mixing preamp was very useful because you switch between 3 separate amplifiers. The combo had the lowest measured distortion available, anywhere at any price! Then my good friend made me switch and compare an ARC D76, with (easily) 1000x the distortion, for a week. That's when I converted to tubes, but I still think fondly back on the Nak system and all the recordings I made with the mixer.
Do you have any thoughts on buying a 610 or a 630 unit as a preamp for a non-Nakamichi system? I think they look cool. It is an idle thought I've had for awhile.
The standard system does have two preamps....the lower unit with the analog meters is a mixing preamp. Very versatile, you can hook up a multidude of items to these without breaking a sweat!