I also have 2 12" kenwood subs - also the £70/£80 ones.
Powered them with a kenwood amp - it was 1 x 1000w or 2 x 300w - cant remember the model number.
Once set up right with the rest of the system, they sounded very well. To be honest, i havent really heard any of the big name brands like JBL or Rockford Fosgate, so i cant really compare. However, i can say that it was enough bass for me - i had a set of vibe components up front and JBL 6 x 9s at the rear, and i only turned the amp up about half way because the bass was just completely overpowering the treble.
I know some people are complete bass-heads, and just like bass and nothing much else. I myself like an all round system. Dont get me wrong i like bass very much, but whats the point in so much that you cant hear anything else?!?
Another reason i bought 2 subs over 1 was that i knew it wouldnt be turned up too much, therefore not stressing the amp or subs so theyd all last longer.