Very difficult to answer,as you'll get many varying opinions and views,but in the end it's going to be down to your ears,system and room.
It's a real problem buying without even seeing what you're getting,never mind trying it,and I do think that with a sub,it's almost essential,although having said that,I did buy an SVS blind,on the recommendations of people here and elsewhere,and felt quite apprehensive about doing so,until it was here and working.
If you really have to buy without trying first,do as much research here,and with as many other sources as you can find(sites like
www.audioreview.com may be useful,as it's mostly consumer led,and with a US slant,it may also be useful for the subs you are looking at).