what you are referring to is called double amping.
while technically it is completely possible to run your configuration this way, its not generally suggested as there will be more distortion than using line out. there are however many people who do thisi.
a few things should be noted:
A) amping then double amping will distort an already distorted signal. you could end up with clipping or other issues. some people will do this for hard to power cans to get extra volume so your question is not exactly odd but audiophiles will never recommend it.
B) the headphone/front speaker out on the back of a soundcard is not high output unless noted. the headphone amp normally works for the internal connection for front 3.5mm jack connectors. if you must use a non aux/non line out connector then this rear jack is the one you would use.
C) if using other than line out push volume to minimum in windows and check your volume output on the magni. you should set windows volume at a level where the magni can push from zero to full listening volume.
D) unless using a strongly amplified signal you arent likely to damage the magni but double amping is not suggested more of a just in case scenario to prevent any accidental damage other than the loss in signal quality.
E) using optical out and using the modi or using a soundcard with analog aux/line out is suggested if quality is desired.
F) the amount of distortion from double amping can be inaudible. honestly it depends on how high of a level the original signal is, the quality of the first amp, etc.
G) as mentioned earlier, an attenuator will reduce a signal to line-level though i dont think it would be required in this case.
H) the dac in the xonar dg is really only entry level so its not a very even match up with the magni. upgrading to a dac as well or using a combo unit like fiio e10k or audioengine d1 might be possible solutions
Thanks so much for your reply, thanks to the others also.
I Guess I'll just go through your letters, seems the simplest way. But before i do I just thought I'd make clear about my sound card the DGX and the headphones I've been looking into.
PCI Express 5.1-channel gaming audio card
Headphones - I'm looking at: 40 or 50 mk3, by FOSTEX, Haven't decided.
MAGNI - I can't seem to find the official page for the MAGNI 1 anymore but, there's a small pdf online the manual.
For the 3 settings for the headphone option I Said originally these are the amplified signal as far as I Know
Instead of using '2 channel / 2 speaker' you select headphone.
then hit the button -
" Select the gain tuning based on your usage scenario of the headphone impedance "
mode 1= <32ohms headsets
mode 2= 32~64ohms headsets
mode 3= >64ohm Headsets
Just to add: I currently own the LINDY HF-110 Open Back headphones
Impedance: 64 Ohm
I set the headphone setting to Mode 3, and use this VolBox I got off amazon, it works.
The 40 or 50 mk3's : 50 ohm .. But I heard they're very hard to power or drive.
// Replies to your lettters //
A: I Don't want to have distortion or clipping
B: I Don't know if it's high output but it's considered an internal amplified signal, Unless I'm mistaken.
C: are you talking about the volume mixer for windows, I pretty much always have them at 100% (right click windows speaker icon / volume mixer ) - Also I'm not using the magni with the Lindys, I use the volbox as volume control. =
D: noted, thanks.
E: Using something else other than my internal sound card is not an option for the source-output, The Xonar Uni-drivers combined with XonarSwitch, is just so convenient, I Can set up profiles for each thing like 2 channel speaker, 5 channel speaker, different headphone profiles etc.. It's nice.
-I've considered upgrading to the 7channel xonar version at some point. =
F: Noted, Thanks.
G: yeah, probly not required. If necessary I just thought about this, I _COULD_ set it to 2 channel speaker or maybe just <32 because that's probly the least 'added on amplification' from the sound card. AND THEN use a headphone amp(?..)
H: yeah, I Don't want to change my source output -I like the internal sound card setup like I said.
What did you mean by 'even match'? not sure, just wondering.
1.Keeping internal sound card or upgrading to 7 channel version (in future)
2.Fostex 40 / 50 mk3 - Hard to drive, when / if I get them - then, I shale see.
write back ssdx when you have time.
sorry for long post.
Hope the additional info helped or clarified things.