A speaker that is rated at twice the amp power rating has long been the safe standard, or the combination of multiple speakers equaling twice the rated power. I personally don’t think that is really necessary though. I have now built 3 6g3 amps and paired them with 65 watt Scumback M75 Speakers. I would use lower wattage but there isn’t one until you get down to 25 watts. The amps have varied in output based on the power transformer used and plate voltage supplied. The lowest wattage of the three was 12.9 clean and 17.4 cranked all the way up with 355 volts on the plates. That would be fine for a 25 watt speaker. The first one I built I used a vintage spec PT and had 440 volts in the plates which gave almost 30 watts when fully turned up, that would not be good for a 25 watt speaker. So from a manufacturing standpoint, a company would spec a speaker that can cover variations like that. Smaller builders may have much less variation or can even spec the speaker based on a single build. I’m comfortable with a speaker that is around 10 watts more than the amp is rated for something lower wattage like a 17 watt amp. I built a JTM 45 that maxes out at 42.78 watts (JTM 42.78?) and I’m comfortable using the same 65 watt M75 in that amp but I probably wouldn’t go lower than that. The last gig I used that amp on I had a 200 watt Eminence Em12n speaker and it sounded great, especially clean. There is zero speaker breakup but the dirty sound is just a little sterile compared to the M75 that gets a little breakup and adds some warmth to the sound. Some may prefer the 200 watt speaker though, to each their own. The amp has a master volume and I turned it down to the level the sound guy wanted, then when I got home I measure the output. I played the gig at 14.5 watts dirty and about 8 watts clean with a 200 watt speaker. 8 watts with an efficient speaker is louder than most people realize and with todays stage management makes amps like the JTM 45 obsolete without a master volume or attenuation. I have a 20 speaker that is going into yet another 6g3 variant I’m building only this is a single channel and uses 6k6GT power tubes and should max out at 10-12 watts and be perfect for most places I play.