remember that subs put out decibels (a measure of sound pressure) you feed them watts (voltage times amperage)
And you feed about any sub 3000 watts and within a very short time you are melting a voice coil...
Thankfully most music is dynamic so the 3kw that the sub is rated to handle would be a short as in millisecond burst of power and the sub better be in the right box etc... But I will try to not over complicate things
Some specs on the box you bought would be nice. the enclosure is going to really dictate the end sound and quantity of bass you get.
Found this on Ebay UK
it's a bit more expensive but a real beast of an amp.
There are some real good amps but again, I need to know coil configuration of the subwoofers to best help you determine what amp.