WHY does one of the tube fuses in my car audio system keep blowin?

Ok, so I had my system installed at best buy and i have two Alpine Type R 12s powered by two Alpine MRP-M500 amps, one for each sub. I have a Fierce Audio distribution block to hold my two tube fuses....the original fuses were the 5AG40A fuse....anyways, the fuse for my left amp blew...took it into best buy, the guy gave me another one of the same kind....it blew again about a week later....took it BACK in. The guy asked what kind of fuse it was, i told him the 40, he said i prolly needed an 80 but he didnt have any so he gave me another 40 and a 60...i put the 60 in...it blew. WHAT THE HELL. The one on the right is not blowing, and everytime the left one blows its fried on the bottom of it...could the bottom clamps holding the fuse not be tight enough er somethin?? the amp doesnt get hot at all so I dont think its defective....suggestions?