Just a brief update, but despite not being ideally placed due to room layout (they are in the front corners), I can honestly say having 2 subs has made a significant improvement.
Once dialled in with Audessey, having a pair has virtually transformed the low end, before it seemed a bit hollow and only at certain frequencies I could hear the sub adding to the sound track and almost separate. Now they sound present throughout, perfectly integrated and adds weight to the audio, making it sound more full/true. It's the first time (even watching a crap movie) its sounded and felt like a true cinema.
Whilst I know its not the most capable of subs, Im happy with them. Just hoping the second hand one doesn't fail any time soon, seems to work perfectly, but when switching on, there's a long delay before it actually switches on and then it switches itself off and then back on. Suspect power side of the amp has an issue, but it works once on, so Im happy.
Next I'll be making some panels for the walls I think, did the clap test and a lot more echo than I expected. So have to work out a design the wife will approve!