You are welcome.
This is quite common. I've got a Porta Corda MkIII-USB and that will also play louder with ASIO than without. Does this mean my USB/DAC sucks? Nah..
The turtle beach card isn't that bad, but a USB/DAC would do better. Something like the Total Bithead is doing better just because you can amplify it as well as get a better signal. It's a completely different price range though, so differences in SQ can be expected.
Just a USB-DAC(without the amp) will make it sound better for the reason that it's a better DAC. Once again it's in a completely different price range.
There are people who are way more knowledgeable on the amp & dac thing so I would ask it in the amps subforum if you are really intending to buy something like that. I think it will increase the SQ quite a lot since the 580 is a pretty high impedance headphone. (300 Ohm I think? - not sure)
Edit: nevermind the last part of this post. I missed the headsave there.
Thanks for the info! I already have an amp...the Headsave Classic with AD8620/AD8610 op-amps that I'm quite happy with. I was just questioning the DAC and it sounds like it would be a good idea to get one. Back into the old wallet!
Edit: Oops...didn't see your edit.