I got this amp mainly as it had a separate subwoofer output channel with its own volume control
I put a Y-splitter cable into the front panel headphone socket coming from my soundcard, one part to the amp, the other to the headphones. The sub out from the amp I connected to this:
Which I screwed onto a piece of plywood that I bolted under my office chair. I'm going to connect the amp and my chair with a curly headphone cable, which will be neater and allow me to disconnect my new electric chair more easily. The rest was a few bits of speaker cable I had lying around, but if you had to buy everything from scratch it would still only be a few quid more. Whole thing took an hour or two to install, a bit of trial & error, hunting through the shed for bolts and plywood etc, and it was a very simple bit of drilling and wiring, little bit of soldering, but in terms of electronics about as easy as it gets.
Nice thing is it just works, you can turn it on and off quickly (amp screwed under my desk) and it works in all games as it uses the sound output. I can't believe I've not cottoned on to this before, it's an excellent value for money add on, really enhances the immersion.
Edit: however, I'm having a spot of bother getting audio mirroring working in FS2 with the rift. There is an option to do this in the rift settings, but it doesn't seem to work. Current low tech solution in VR is to unscrew the rift headphones and use my normal ones, until I figure it out. Works fine in 2-D no probs as it comes through the headphones anyway.
Edit edit: Phil's solution looks good, I'll try that