Headphones and the Devialet

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If I understood correctly, you're already using balanced output to your headphones, as that's independent of what goes into the headphone amp.

If you want to keep the 200 in the chain, then you'll have these options:

1. use RCA-to-RCA cables from the 200's stereo analog pre-amp outputs into single-ended analog inputs on your headphone amp
   
2. use RCA-to-XLR cables ("passive adapters") from the 200's stereo analog pre-amp outputs into balanced analog inputs on your headphone amp
   
3. use RCA cables from the 200's stereo analog pre-amp outputs into an active or transformer-based single-ended to balanced converter, then XLR cables from the converter into balanced analog inputs on your headphone amp
   
4. configure your 200 to produce a digital output on one of the RCA sockets then use RCA-to-RCA cable from that digital output into a co-ax digital input on your headphone amp
   

Option (1) is what you're doing at the moment, if I read your posts correctly.  Pretty straightforward and involves no extra boxes or unusual cables.  The fact that your headphone amp receives a single-ended rather than balanced input is probably of no real consequence for sound quality, unless it does something strange internally.

Option (2) is (I think) only worthwhile if your headphone amp has a noticeably different sound quality when using the balanced inputs, which would be unusual and surprising but not impossible.  RCA-to-XLR cables are not too hard to find but it does add cost especially if you like "boutique" cables.  An alternative would be to use RCA cables with a pair of something like the Neutrik RCA-to-XLR adapters.  Bear in mind that this still doesn't really give you "balanced" operation but is just a way of using the balanced inputs to your headphone amp with what it still a single-ended signal from the 200.

Option (3) is would give you something closer to balanced operation although at the cost of the extra adapter boxes, which might need to be powered (if they're active rather than based on transformers).  This is going to be more expensive again because apart from the cost of the adapters you'd also need both RCA cables between the 200 and the adapters and XLR cables from the adapter to your headphone amp.  As mentioned previously the adapters may themselves affect the sound quality so it will be hard to assess whether this is an improvement until you put the whole thing together.

Option (4) is probably the simplest and most effective approach provided your headphone amp has a co-ax digital input.  With this approach you don't have to worry about balanced or unbalanced input to the headphone amp.  Sound quality will depend to some extent on how the DAC in the headphone amp compares to the pre-amp output DAC in the Devialet.

In short, I'd suggest either option (1) or option (4) depending on the capabilities of your headphone amp.

Looking at the specs for the Burson Conductor 3 series helps to narrow down the options further:

• only the model 3R has line-level analog inputs (on RCA connectors) so you'd need to use that model if you want to use option (1)
  
• only the model 3XR has line-level balanced analog inputs (on XLR connectors), so you'd need to use that model if you choose options (2) or (3)
  
• all four models have co-ax digital input so could be used with option (4)
  

If I were going to build a system with an Expert 200 as a "pre-amp" to a Conductor 3, I'd choose option (4) unless Burson strongly advise using analog inputs, in which case I'd use option (1) which would limit my choice to the 3R model.  Option (4) would allow any of the Conductor 3 models to be used.

Also, all four models have a USB input so could be connected directly to your Nucleus if you wanted to take the 200 out of the chain.

Hope that's helpful!