The Delius is meant to be run directly into an amplifier unless you have a SUPREME, megabucks preamplifier like the Ayre KX-R or something like that. It has a superb preamplifier on board. It is very transparent. If you don't like what you hear, you need to season with source or downstream components, cables, power regulation and noise reduction equipment,ect. No additional preamplifier is required. Most will do more harm than good and simply aren't good enough. Make no mistake, Purcell/Delius is some of the best digital on the market and there is a preamp onboard to match it. My Delius replaced a $6,000 Threshold preamplifier easily.
However, Purcell/Delius demands the finest of everything to reach its full potential. Finest digital cabling, finest power conditioning, finest source, power supply, you name it.
Rather than waste time with adding a redundant preamplifier, if you don't like the present result you should do the following to unlock the performance potential of your existing setup:
1. Upgrade the power supply to your Duet using a Channel Islands made unit. I did this for Squeezebox and it was a tremendous upgrade for $250. Night and day. Then upgrade the power cord on the Channel Islands unit to something like a Transparent Audio power cord.
3. Absolutely use the best S/PDIF cable you can afford to get the information from Duet to your Purcell. Get a Transparent Audio Reference Digital Link if you can afford one. (it is the red/purple one).
4. Upgrade the power cables on your dCS units (and source PSU) to the best you can afford (I recommend the latest Transparent Audio power cabling). It is amazing how dramatic the improvement can be.
5. Upgrade your interconnects from the Delius to the amp.
The Purcell/Delius combo is a TREMENDOUS platform you can build off of for years. Unless you have optimized your cabling and your other components, you'll never know how good a Squeezebox/Duet/dCS combo can really sound. The dCS equipment responds remarkably well to things like power conditioning and cabling.
You may get to a point where you have maximized the rest of your system and adding a preamplifier makes sense. But you will probably be surprised to find that making these less expensive optimizations brings things to level you didn't think possible.
This advice is based on my optimizing my Logitech Squeezebox/Delius/Purcell/B&W 802 setup. I added a Channel Islands Power Supply, two new Transparent brand power cords, and a four PS Audio Noise harvesters and I have a completely different system, smoother, more natural and listenable, ect. Before I did that I was considering adding $5,000 word clocks and preamplifiers and you name it. These tweaks got my system where it needed to be. The Delius truly is something special, preamp and all, but you have to build a system that is worthy of it.
You can get away with the Duet but you absolutely have get the updated Channel Islands power supply. You have to remedy that deficiency first or you will be on a wild goose chase with that as your source.