Do you have to have a headphone amp? Do you listen to speakers as well or would you like to have a good option for that for possible future use?
The reason I ask is that while the "HT | OMEGA Claro Halo" has a quality Headphone Amplifier It doesn't have 5.1 or 7.1 analog connections and if you want to use your cards high quality DAC's (digital to analog converter) for PC speakers greater than 2ch you will need the analog connections.
There are several options. You could buy the "HT | OMEGA Claro Halo XT" or add on the "HT OMEGA XT Extension Board" at a later date assuming that HT | OMEGA still sells them. Right now they are available direct or through amazon for $59.99
http://www.amazon.com/gp/offer-listing/B005OU7MZ2/ref=pd_sxp_f_avail_used?ie=UTF8&redirect=true&condition=all
Of course this is assuming you have the need, desire and/or room for it in the first place. The Halo and Halo XT are excellent cards by the way.
You could also forgo the Headphone Amplifier if it's not super important and buy the HT | OMEGA CLARO II 7.1 Channels 24-bit 192KHz PCI Interface Sound Card
http://www.newegg.com/Product/Product.aspx?Item=N82E16829271007
But the best option for you might be this card: "HT | OMEGA eClaro 7.1 Channels 24-bit 192KHz PCI Express x1" this card is very low profile and should fit nice in your top PCIe x1 slot which is were I have mine.
http://www.newegg.com/Product/Product.aspx?Item=N82E16829271006
These last 2 cards don't have a dedicated headphone amp but, my A700's still go pretty loud on my eClaro and they do have both digital and analog connections for up to 7.1. All of these cards, the Claro Halo, Claro Halo XT, CLARO II and the eClaro use the same Audio Chipset. They also all have a five egg rating on Newegg. There isn't even one single Asus or Creative internal sound card that can boast that.
The Claro Halo, Claro Halo XT and eClaro also have a swappable OPAMP socket.