Hi Guys

I just order from Amazon 2 M cube speakers to use as back speakers,
and i saw that they work on 280HZ - 20KHZ
Now how bad is it? if i will have a sub to support it, and i got B&W as front.
it will do the job of back speakers? effect sound? or it will sound like "old radio"?
is it better to get small back speakers that go lower like 100hz ?