I own a Rythmik F12 sub to pair with my KEF system,Is a sub that i never regret.The sub is well combo with my KEFs and it gives me a good clean and fast bass output. I am sure u will be more on an  eye opener once u had the taste of different sub manufactures to test around compared to the Q400, then from there i am confident to say,u will find the sub that suite u for your setup. When it comes to sub,the poison is very very tempting.Beware !!

---

Added on May 27, 2012, 7:52 pmYo terranova bro; so long no hear from u.How's your current system going,any new upgrades on 2012?When is the AV convention in HK this year?

My room is W 10' x L 20'. I had audition both F12 £ F15, performance would be a significent diff for the F15 , it delivers stronger and tighter LFE.
From your room size a F12 would serve u great but if budget permits, the F15 will give u more headroom experience.

if only junwei can lend us some subs to test for free, all will be poisoned