It's impossible to diagnose such problems over the forum. However, this amp needs to go into a service/repair center. Problems which occur suddenly are seldom specifically related to a tube. And seven months is far too soon for all your tubes to have aged to the point of replacement. You should get several years of tube life from a Mac amplifier. You don't give an indication whether this is the total life on the amp or whether you bought the amplifier second hand.
The problem is most likely related to an internal component which may have taken out a tube. First, check that all the tubes are working. Darken the room and look at the glow from each tube. All should appear equally bright and the same color. If one or more tubes do not appear the same as the others, that is the beginning of your problem. The real problem probably lies in a bias resistor or power supply cap which has gone astray. Hopefully, it hasn't damaged the tube but only a technician can judge that by putting the amp on a test bench. Take the amp into your Mac repair shop and the cost for the parts is very likely to be minimal. Do not replace tubes until you've had the amp checked. Putting new tubes in a bad circuit will only give you one more bad tube should the problem be severe. Macs are built so that it is unnecessary to replace all the output tubes when one tube goes out. If one tube has gone kaput, you should only need to replace the one bad tube.
I repeat, do not put a new tube in the circuit until you know the circuit is operating properly. You might consider buying a spare set of tubes for this amp to use a quality check. Buy something very generic and as low cost as possible. You just need tubes that work not work well. You can then use these tubes to help diagnose whether the problems which occur might be in the amplifier proper or merely a tube that needs replacing.
Shall we rub it in? It's not the tubes?