@maprik, to directly answer your question: Could having just one subwoofer (REL T/7x)with the crossover set around 60hz pull my image to the side that the sub is on?
Rereading my long post I realised that I did not specifically address this concern of yours so I apologise for that. If the sub has been properly set up regardless of where it is positioned it will not skew the image, the variable phase adjustment/timing and smoothing of the response will deal with this and this is why I try and impress upon everybody the importance of being able to measure. It's not expensive nor difficult to do and there are many youtube tutorials and articles on how to proceed. The best way forward is to read articles from the likes of Toole, Welti and Geddes. Dr. Earl Geddes actually wrote his thesis on this very topic and I suggest you take heed of their teachings. The posts in this thread are I'm sure all well intended but for someone new to acoustics it would appear confusing and daunting. It's not. It's thoroughly investigated science and proven in practice and all within your reach.
The cost of the mic. which is about the same as a meal for two will be the most valuable cost effective purchase anyone can get and is IMO the only way to dial them in. The measurement system does a lot more than help position the subs it will provide a waterfall plot of the time it takes for the sound to decay by a certain amount. If your room has a lot of hard surfaces and bare windows the sound from the seakers will take a long time to deay creating a confused smeared image with loss of detail.
This is scientifically based and not subjective in behaviour. It is difficult to conceptualise the damage strong early reflections cause which can be dealt with by placing some broad-band absorbers in the right place. Again measurement is necessary where the decay is computed for you by the amazing App REW which will provide the time it takes for the sound to decay by 60dB across the spectrum known as RT60 usually about 300ms for domestic listening rooms. RT60 depends on the intended use and volume of the space. Tables can be found online and the times will differ depending on whether it's for a lecture room, school, church etc.
Getting a smooth frequency and decay response is paramount and I state with confidence that a modest system dialed in as per the above will get you a more immersive and rewarding experience than a big $$$ system not dialed in. All for the cost of a mic. and if necessary which it usually is some absorption.
Hope this was helpful and good luck.