Best as separate units, unless you really prefer the sound of the 4140's amplifier. I'll not delve into the Sansui versus Marantz debate (both are excellent makes, each has their die hard fans), but the 9001 lacks pre-out/main in facilities so taking a preamp signal out to feed a different amplifier will require modification to add such connections.
The quad decoding and synth capabilities of the 9001 are exceptional, while most of what I've read on the subject (to be clear, I've not listened to any Marantz quad hardware myself) has indicated the Marantz quad decoders are underwhelming in terms of how good a 4 channel effect they produce.
All that said, any 9001 in unrestored condition will almost assuredly not be up to par. It has one particular problem with bad solder joints for vias (pass through joints) on a key circuit board and all of these need to be redone for proper passage of signal on all four channels. This is just the most glaring issue that will need attention, the driver stage needs minor corrections due to age related failure of several components, the quad decoder stage will need a proper alignment.
Further, given these are 40 years old now, they should receive a full re-capping. Much has been written here over years now on what needs to be done, the search facility will bear out many threads on the 9001 and related models, and what work should be done and how to go about it for the DIY'er, though I'd not recommend taking the job on as a first effort. It is a very complex unit and access to some parts is difficult without lots of disassembly.