I have my DacMagic now. I'm still in the process of breaking it in so I won't do a serious 840c v. DacMagic comparison just yet. Also, I got distracted by comparing my DacMagic to my Trends UD-10.1 as a USB->SPDIF converter. Sorry if this gets a little off-topic, but some of you considering a dacmagic might be interested in what I'm about to say.
So far I have used the following speaker setups for my comparison:
flac>foobar2000>asio>usb>trends ud-10.1>840c dac>emotiva rsp-1 preamp>d-sonic 1000s amp>paradigm s6 and jl audio f112
flac>foobar2000>asio>usb>dacmagic (used as usb to spdif converter)>840c>emotiva>d-sonic 1000s>paradigm s6 and jl audio f112
I find it easier to spot differences in components using my speakers than my headphones. I'll still do a comparison with both headphones and speakers when comparing the dacmagic and 840c as dacs, but for now the dacmagic and trends are being used interchangeably to pass a digital audio signal around my living room to my 840c which then does the dac stuff. I was expecting the dacmagic and trends not to be changing the sound at all. I'm surprised to say the dacmagic and trends sound different in this configuration, and the trends is definitely better.
I'm listening to my favourite demo music: track 2 of The Guitar Trio by Paco de Lucia, John Mclaughlin, and Al Di Meola (ripped from my cd to flac). With the dacmagic acting as usb converter, bass tones exhibit a mild ringing as if they are being slightly sustained. The reverb on any tones which already have a reverb effect in the original recording are also more pronounced. The difference is a little on the subtle side, but I can definitely notice it. I flipped between dacmagic and trends a good dozen times before I could fully explain the difference I was hearing. USB audio converted through the Trends sounds more correct. The bass tones and reverb are crisper. They sound much closer to the same track from the CD as played in my 840c. Unless the dacmagic changes during break-in, I will definitely be keeping my Trends to be used to convert usb audio to spdif in my system.
My preliminary conclusion is that the usb implementation on the dacmagic is inferior to the trends. Possibly the dacmagic has more jitter when connected through usb. I don't know what jitter is supposed to sound like though. For anyone more knowledgeable, does jitter sound like what I described above?
Note that when playing the CD in my 840c, it's ever so slightly crisper than when using my Trends for usb conversion. The bass is cleaner when playing the cd on the 840c. I'm definitely not imagining it. Perhaps the trends is introducing some jitter too, just much less than the dacmagic.
This testing process is making me wish for an asynchronous usb to spdif converter.