It seems your speakers have separate connection for each channel. Most if not all AV receivers, that have analogue input, use separate connection for all channels. Sound cards always combine sub and centre, just like left/right and rear left/rear right are combined. If this is the case with your speaker system, are you using cables that split the 3.5mm stereo jack into 2 RCA/phono connectors?
Having a sound system that uses analogue connection, rules out the Essence, as that is a stereo card, same as the Titanium HD. The Xense allows for connection of up to 7.1 speakers via analogue, but that's only available as a bundle.
I'm going to stick my neck out here, and say I very much doubt the speakers you have currently will make use of what a £150 sound card is capable of. The wattage and power of the speakers has nothing to do with the sound quality, only how much volume there is. Neutac? Never hard of them. They are probably fine to a point. The fact that you have better speakers in the Onkyo/Jamo setup, gives you a better understanding, as you can compare the Neutac's to something better. Someone who owns the Neutac's, and has had nothing else, their opinion doesn't mean a lot. Without comparison to something better, anything can appear to be great. It's only when we can compare things to better products, that we can appreciate how good or bad something actually is. Obviously, you feel there is room for improvement, by getting a better sound card. That's understandable.
The Home Theatre HD does look nice, but it's bloody expensive, and somewhat unnecessary. For sending audio via HDMI, an AMD/ATi 5/6 series GFX card can do that. Unless you plan to buy something which will use HDMI from it, you are paying for something you will never use. I would look at something like Xonar DX, D2X or a X-Fi Titanium. They are all better cards than the Xtreme Audio, which isn't really an X-Fi at all, it's a repackaged older card.
