To be fair, you didn't really check. It was more by good luck alone that you just happened to mention your plan.
Work-arounds..... this is when we start to try to make a silk purse out of a sow's ear. These things never end well. Bearing in mind how I just saved your sub from going up in smoke then I hope you'll take this piece of advice and just run with it.... Messing about with a work-around is a waste of time and money. Yes, it can be done, but it involves many compromises and is rather short-sighted.
If I was you I'd sell the complete HTR-390 kit and buy a proper AV receiver with a subwoofer out. After all, you won't easily be able to shift the Onkyo speakers on their own, so they're going to sit gathering dust until you have a complete kit to sell.
Rather than dropping £50 of dead money in to some workaround boxes why not advertise the 390 kit for around £150 and pick up a used AVR of similar spec for around £100. The TX-SR333 would do just fine. There's one on AVForums classifieds right now. If you want to really give those new Q Acoustic speakers a good workout then there's a Yamaha RX-V1800 for sale at £200.
Points taken. However I was not just going to connect the sub without double checking first, purely because I had my doubts about it being flawed logic.
Thanks for the info about the units for sale - will look into them.
Thanks again.
Mark