I've realised that I can connect my STB to the through port on my DVB-C card, and it still receives an excellent signal. I conclude that my cabling is sufficient.
DVBShop say that the Linux drivers don't actually report signal strength correctly. I don't know if that means that they somehow diminish the actual signal. It seems unlikely, but I'm no expert.
I suppose I'll have to pull the card out again and put it in a Windows machine to see if the signal is significantly better.