Hello m0xpd,
first of all I would like to say thank you for your library and the description you've posted. Great work.
Easy to read, easy to follow and it also got me a watering mouth while reading it.
But now I have to bother with some questions that I would like to ask you as you seem to be "expert enough" to have all the answers. 
As I consider to create a musical instrument, a synthesizer, I ask myself if this, the chip as well as your library, would do the job.
I dont know that much about how to do anything of the things I have to do, but I am keen to get it done.
Right now my brother and I build a Arduino based 16 step sequenzer, and wihle researching for related stuff I found a website that sells a AD9850 breakout board which isnt that expensive. http://www.ebay.de/itm/281274553982 Or is the a "better" one?
Questions I have are
- can I use this as I could use a "industrial produced musical instrument"? Wich means
- does the chip has the capabilities to generate envelopes to produce complex sound?
- how to edit the parameters if the chip is capable to do that? With potis attached to one of the pins and via shift register or such?
- would I need an amplifier on the outputs of the board to connect to a line mixer?
- does it really make sense to invest in a DUE if the questions above can be answered i a positve way?
- is the more coffee?
Thank you for your time.