When I wrote my review, I was using the stock Hiface as a transport paired with the stock (and older version) of the DAC. The combination was excellent and you didn't feel like you were listening to an entry level set-up. While it wasn't as detailed as some of the more expensive DACs and CD Players I tried, it was relatively natural sounding.
A little anecdote: while I was audiotioning headphones before getting my Beyer T1s, I took my C2 headphone amp as well as my HD650s to a high-end store. The CD Player they were using cost around 3000€ I think. While that CD player had a bigger soundstage and was more detailed than the Purepiper (that I still had at that time), it was artificial sounding on some test CDs I brought with me.
However, I can't say the same for the USB to SPDIF module I tried: It was even warmer sounding than the Teralink X and lacked overall resolution. While it does the job of transforming 16/44 and 16/48 data to spdif, it has limited performance. I would rather recommend the Teralink X2 (which has 24/98 support) or even the Musiland 01 USD over the Purepiper's one. But to keep things in perspective, I think they were selling it around $15 so they obviously provide it as a temporary means of connection.
With that being said, the Purepiper DAC A-1 is less sensitive to the quality of the transport (and the power supply for that matter). The FUN, in comparison, is far more sensitive to the quality of the transport (as well as the power supply).
I was told that its jitter rejection is due to some special set-up of the DIR9001. If I can find back that email, I will update the post.