As long as the received signal to noise ratio is not compromised, the efficiency of a receiving antenna doesn't much matter because of the 70+ dB dynamic range of the receiver AGC circuitry. However, the efficiency of a transmitting antenna certainly does matter and 70+ dB of gain at the transmitting antenna, even if it were legal, would probably melt any low-efficiency antenna. How efficient is your antenna-in-a-box? How much power can it dissipate without overheating or melting? Or looking at it another way - Assuming you are listening to an S9 signal, how much power do you have to deliver to your antenna-in-a-box for your signal to be S9 at the other end?