What is safe? An RMS rating tells you the power you can output for prolonged amounts of time before a component dies from heat. This power level should usually be quite below clipping level, so basically it tells you which component will die first when continuously exceeding its rating while staying outside of clipping range. When amplifier and speaker are reasonably matched, this is not as much a question of whether their combination is "safe" rather than whether you are reasonably matching the power requirements of your venue with either.
Now when clipping gets involved, things change. When solid state amps clip, their signal spells death for tweeters. So you either want vastly overdimensioned speakers, or you want the speakers to be the factor limiting total loudness in a manner causing the DJ to revert to otherwise unheard-of measures, namely turning down the volume (so that the amp never gets to be the limiting factor). Assuming the DJ has enough hearing left that distorting speakers are still accessible to it, of course.
So usually your first priority should be matching both amp's and speakers total output in scale to the venue. And when they are comparable in RMS and of sufficient dimension that you are reasonably sure they won't die the heat death from continuous output of RMS power, it's not actually a cause for worry that the power ratings of the speakers are somewhat below that of the amp.
Now those are basically PA considerations.
You are talking about an instrument amp here. Those usually are driven closer to their limits. Instrument cabinets tend to be more resilient to tweeter damage (you probably don't even have tweeters) and overdriving a tube amp is part of its sound.
Instrument speaker heads/cabinets at 1000W are silly in my opinion. At those power levels, you are talking about PA ranges of reach and venue, and balancing a band mix acoustically at those power levels rather than microphoning them and balancing at the PA level does not work all that well.
Much of your audience will then get to hear a mix that is quite out of balance. So if you are using your amp in any sane manner, you'll not be driving either head or cabinet anyway near their limits and matching their RMS rating is quite an academical feat.