You will, after some use, need new tubes for the amp. So, buy a new set now, regardless of whether or not the current set is worn out. Once in a while (say every six months or so), replace the current set with the new set and compare the sound. When the difference is in favor of the newer set to the point that YOU think it is worth the cost of replacing the tubes, make the switch and buy another set of backup tubes. You should also test the power (output) tube set separately from the smaller signal/driver tubes since the power tubes tend to require replacement at an earlier date. Having extra tubes will also mean that you have a backup for the occasional premature death of a tube.
