@mikedc you’ve received a lot of good advice up above. And some good solutions proposed. I’d like to suggest an intermediate step before you go shopping for new speakers or a new 300B amplifier.
I humbly suggest you download (if you haven’t already done so) an app for your phone like Decibel X PRO that will allow you to measure what sound pressure levels you do most of your listening at. This will provide some empirical data (yes I appreciate that it might not be as accurate as it would be if you were using a calibrated microphone, etc. but it will get you started) so you know approximately how much power you are using to listen to your music in your listening room on your speakers. The version of Decibel X PRO I use costs a couple of bucks, but I find it very convenient and use it frequently. I expect there are alternatives out there that would also do the job.
Personally I would seldom be listening at 86dB. That’s just me, in my listening room. But for me, 86dB is LOUD. Yes, I might sometimes feel like cranking it up above that level but I find that most of my listening is at a lower and more comfortable level. You need to find out how loud you’re actually listening.
I bring up the 86dB because you said your speakers are 86dB efficient. That is to say, if I understand you correctly, that they will produce 86dB with one watt at one meter. Yes I appreciate that I am making some generalizations here and that nobody sits around listening to pink noise... But you might find that you could dip your toes into the 300B water without changing speakers or buying an expensive parallel single ended or push/pull 300B amp. You might be surprised at how loud your speakers will play with just a few watts.
Yes, of course, more efficient speakers, like a pair of Klipsch speakers, or vintage Altec VOTT or whatever would be a better match for a "low powered" 300B amp. But I’m simply trying to suggest an action that will allow you to have a better idea how much power you need before you go out and buy an expensive amp or change your speakers.
BTW, I no longer consider an 8 watt amplifier to be low powered. I’m now routinely listening to my 94dB efficient speakers with just 1.5 watts. It sounds wonderful. Of course, my music, my room, my tastes. And YMMV.
If you like to listen Jimi Hendrix, or Motorhead or Mahler at high levels (say, at 95 - 105dB) then 8 watts isn’t going to do it for you with your current speakers. But if you’re listening to a wide variety of music at lower volumes then you might be surprised.
Another comment. There are many alternatives to SET 300B amplifiers. And the devil is in the details. And I’m sure there are crappy SET 300B amplifiers out there. Just like there are probably compromised PP 300B and PSE 300B amplifiers. But there’s a reason people consider SET amplifiers special. It’s that first watt. Maybe the first couple of watts. And the simplicity of a well executed SET circuit. When you start making the design more complicated to produce more power you are going into a different direction and there are many wonderful alternatives to the 300B that can sound great. Another way to say this is to say that the 300B is simply one alternative tube. And a SET design is simply one approach. There are other wonderful triodes out there and some of them make a lot of power and can sound great.
One more comment. All equipment is a compromise. Cost, size, appearance, power, efficiency, "wife acceptance", etc. The key is synergy. That’s where all the accepted wisdom and advice reach the limit of their usefulness. Your direct experience as you go forward will be much more useful in this regard than anything you read on these forums. Good luck in your endeavors.