I used Vibe slick 6's which were about £40 from halfords at the time but seem to have gone up a bit now..
there's a pair on ebay for less..
(link) depending on if you're trying to keep things to a minimum cost wise.. you could order 2 pairs and just do all four doors for £60
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but hang on sorry I just need to backtrack slightly, have you got the amp under the seat as well, and which cables did you use to connect to your new headunit, did it involve this convertor by any chance
https://www.ebay.co.uk/itm/171167300967?hash=item27da5dbd67:gKcAAMXQUmFSesW 6 with the black plug connector and the RCA leads
if not, then before you order anything tell us what the output rating on the headunit is (should be in the handbook) or what model headunit it is