Advise on adding sub to 2.1 Setup

Hi All, 

Currently very much a newbie to the audio world and am seeking some advice. 

I currently have the KEF LSX hooked up to my Computer via an optical cable, and whilst the sound is very good for active monitors, I want to add a sub to enhance the low end. 

I want the sub purely for music, it will serve no purpose to me to watch movies or anything else. Whilst I want the low end to be there, I want it to be as seamless as possible, and enhance the sound of the whole system, but also able to match the speed of the LSX.

I wanted to know the following 

-> What subs could you recommend with the above spec, my budget is really about £250 at the most, unless I'm told that I'm way off that with above spec, in which case lets move to £500, and I'll get saving. The size of my room is 24 meters. 

Unfortunately, I am somewhat tied in terms of where I can put the sub, and it would have to be under my Computer desk (Pictures attached). I do realize that this isn't ideal in terms of placement, and that the crawl test would provide optimal positioning however the mrs's would have something to say about that as the room is quite open. 

-> Do I *NEED* a certain type of sub? For example at my dad's home, he has an amp for home cinema, which the sub is connected to, whereas I am connecting directly to my Computer via Optical. So whilst there is a sub out port on the Master speaker, are there certain sub's that I cannot directly port into?

My First post on this site, so I appreciate everyone's patience, and look forward to hearing from you guys :).

 

What sub is in the same range of kef speakers you already have? 

Bump?

Since when was the F1 218 a sub? 

DNbshaggy wrote: You should get a f1 218