I'm not trying to rain on your parade but for £89, that amp has no chance of the power output it shows. Only a perfect amplifier will double the output when the impedance is halved. Car audio doesn't need to conform to many specification guidelines and this one clearly doesn't. Also, it's not one of the major brands, so it will be hard to prove and I suspect, get service from the company.
This isn't something I would buy sight-unseen or without a demo. Unfortunately, it's not always possible to see/feel/hear equipment but it is necessary if no review is available. Reading an ebay listing isn't the best source for information. I have no idea who FLI is- have you seen much of their equipment before? Also, the curved top on the sub box doesn't matter when it comes to sound quality.