There are 2 things you should now do, if you plan to keep running it like that:
1) Choose current ratings for HT and mains fuses that are half what they normally are. Using half the output tubes cuts the current draw of the amp roughly in half (preamp tubes do draw some current but the most of it is for the output tubes). If you use the normal values of fuses while only running half the output tubes, you're risking more potential damage to the amp if those tubes fail since the fuses won't fail until they're handling twice the current they should (under the "half power" condition). It's not a huge risk, but it's worth doing especially since fuses are cheap.
2) Double the impedance. That is: hook a 16 ohm cabinet up to the amp's 8 ohm output (for example). This will provide a closer impedance match when running half the output tubes. The amp was designed with a transformer matching 4 output tubes to a load. Running it with 2 tubes changes things. Doubling the impedance like that compensates for this. It'll sound more normal and be safer in general.
Also make note of which output tube sockets you aren't currently using (sitting vacant). If a problem occurs later when you use a matched quad of output tubes again, it's likely one of those 2 sockets has a failed screen grid resistor. Making a note of this now is a good idea. It's easy enough to test for, but it can't hurt to make a note of it.
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