5.1 System questions

I have a Xonar Essence ST card with the 5.1 daughterboard, and have been using this with a Creative Gigaworks speaker set for a few years now, but the speaker system has a high pitched whine, so I have decided it is time for a change. I sometimes use the card to power a set of Sennheiser earphones but they don't get much use to be honest.

I have found a nice AV receiver and 5.1 speaker set that I am probably going to go ahead with (unless anyone here can recommend alternatives?), but my main question is around the sound card. Looking at the following kit so far...

http://www.richersounds.com/product/av-receivers/marantz/nr1506/mara-nr1506-blk
http://www.richersounds.com/product/speaker-packages/tannoy/hts101xp/tann-hts101xp


Is it worth keeping the Essense ST card if I was using optical to power the av receiver, or is there a better alternative? thinking of upgrading motherboard soon to Z170, would onboard deliver as good a sound over optical as my current sound card, and are the onboard powered headphone amps any use?

I use my ST via optical into my marantz AV7701. If I want to have full 5.1, I switch the digital out from PCM to dolby digital live. Technically it's wasting some of the potential of the card as you're no longer using the high end dac but in my case at least, I still use the ASIO support a lot and occasionally the builtin headphone amp. Plus live 5.1 encoding via bitstream (ala DDLive and DTS Connect) is not available on all sound devices. I've only just recently removed my H6 daughterboard too as I've never made use of it.

You would either use HDMI or optical. That probably depends on what you are going to use the speaker set up for and what kind of monitor/TV set up you have.

If you are going to use optical, then as Kei mentioned, you'll need to use Dolby Digital Live on the Xonar Essence ST.

You could ditch the ST and use the optical from the motherboard, but that would probably something only really worth doing if you are looking at motherboards without a PCI slots; otherwise you might as well keep it, especially if you are going to use headphones. You'll get better audio output from the Xonar for headphones than any onboard audio; although admittedly, the Essence ST/STX isn't the best choice for using earphones due to the high output impedance. Still going to be better than using onboard audio though.

Edit: Forgot to mention that your choice of AV receiver and speaker package look good to me. Marantz make some very nice gear and the Tannoy TFX is a good budget 5.1 speaker package.