Motherboard SDIF Output Or Soundcard

Hey everyone, I'm using my computer as a source of music. I'm really happy with the sound. Just wondering if it will be worth it to buy a higher end sound card. What do you guys think? 

Rig: Running an Asus X99 Pro Motherboard. Using a 6 meter long Audioquest toslink cable from motherboard to Musical Fidelity V90 DAC, to Parasound JC2 Preamp, to a Parasound A21 amp, and finally, to Von Schweikert V4.5 speakers. 

Sound cards might reduce electrical noise, that can convert to time jitter of digital signal (especially with slow Toslink transitions).  This jitter appears as added noise (loss of clarity).  On the other hand Stereophile measurements of V90 show very good jitter rejection.  V90 also has asynchronous USB, that might provide better results (2-wire, no power USB cable).  Both, Toslink and USB are limited to 96kHz.  Again Stereophile measurements showed similar very low jitter with both methods.  If you're really happy with the sound, the leave it alone and enjoy.  One possible area of improvement is clean power supply.  My Airport Express, DAC and Power Amp are all plugged into Furman Elite-20 PFI conditioner with very good noise filtering.
At the moment I too use my computer as a source. I felt it best to avoid the sound card altogether and output via USB.  
+1 gdhal.
I, too use either USB or Spdif and output to a Schiit DAC.
One upgrade it did recently was to get Fidelizer Pro. It really made the sound reproduction much better.
As with gdhal and gdnrbob, I also use USB. I would consider the purchase of a used M2Tech USB to SPDIF converter which plugs directly into the USB port and run coax to the V90. No USB cable needed. This opens the possibility of purchasing a older used DAC having a more robust power supply and analog output stage as a further upgrade. 
Great suggestions guys. I want to one day own a NAD m51. There was one used on eBay for half the retail price not too long ago. But I was a bit too skeptical about purchasing something so expensive used. 

In the mean time the v90 is a little gem. The sound I'm getting is excellent. Especially with modern day recordings. Although it does seem to lack a bit in the older recordings department. It's missing what vynl has to offer, warmthness and richness. But hey that's what my turntable is for, for all the old recordings. 
I think you should try a Schiit DAC. They have a trial period, so all you would be out is shipping costs. Great performance at a low price point.
If you do, I would love to hear what you think.
Bob, I checked out their website. Very handsome electronics. I like the case of the bit frost. The electronics look very nice. For some reason, Schiit reminds me of a hand made product built by one person, who eventually got big enough to hire people to build even more all while continuing the passion for using quality parts. That's the vibe I'm getting from their website. And with a name like that, it can only mean they aren't stuck up, which might mean products that are more accessible to anyone. 

I am intrigued by the bit frost multibit.. 
Yes, Joe, they are an interesting company. Though I don't understand their rationale for not providing information regarding the sound qualities of their different models. After all, if they are doing something between models there must be a reason-hopefully, something that relates to increased sound definition.
In any case, I own the Bifrost Multibit and the Modi Multibit, I am very happy with both. I really can't see how much better the more expensive models could be, but I wouldn't doubt it. 
Now if I can just find a used Yggy...
I agree that the Schiit products offer great return for investment. Can't help to think that the Bitfrost Multibit would prove to be an improvement over the V90. Especially for the playback of redbook CDs as uncompressed computer files.
I would recommend an USB Interface and a USB isolator.  I use a Schiit Wyrd ($99) to isolate the USB signal from a noisy computer port and then a Gustard U12 ($160) to convert to spdif.  The Gustard has dual TXCO to minimize jitter and it sounds fantastic.  
gdnrbob:  Yes there is a significant difference.  The sound is tighter with a lower noise floor.  USB ports on computers are VERY noisy devices and the Wyrd repeater helps.  What I like about it is that it uses a linear power supply (the adapter is AC) and then has precision regulation inside the unit.  It worked so well for me that I ended up buying a second unit to be used at my office.  For $99 it does its job well.  Even high-end dacs can benefit from it.