Speaker for 300b amp?

Check out the ampsandsound website.  If you see something interesting (or even if you don’t) give Justin  (the Owner) a call.  He usually answers the phone and can be very helpful on the subject of 300b amps and matching speakers.

OP 

I have heard the Reference 3S deCapo powered by a pair of Mono 300B

it sounds sublime, very musical and it truly engages emotion. the DeCappo do not have a Crossover and only uses a single Cap on the HF Driver. It is a stand mount speaker, if you are looking into this size

Larger speakers are best served by Pure Audio Project (PAP) which i also have heard. The DeCappo , for me, needed a well-integrated Sub to fill in, while the PAP can be stand alone. Your choice

Btw, I also like Zu speakers and Ryan Casey @Marlin38.  I had a pair of Dirty Weekends for 4 years during COVID and they really performed well with the Cary 300B. I always thought they punched way above there price level but unfortunately sold them to make room for Clayton Shaw's Caladan's which also played well with the Cary but was not a great fit for my smaller room.  Harbeth HL5XDs followed the Caladan's but too much bass bloat for the room and not efficient enough for the Tool Shed amp.  As stated in the above post my search for that room has ended with the Omega's. 

mikewarner

You did not mention price range

Volti or Fyne - Both brilliantly made, very high sensitivity, and sound great with low wattage amps with some current behind them

Volti Luceras are 99db efficient, and the Razz are 100db

Fyne Vintage classic line is around 95 db, the Vintage Gold line is 96/97db, and the Vintage are 96/97 db. The price 

Earlier in 2025 I adopted a SE 300B Power Amp’ and Floor Standing Coaxial Driver Speakers, under the guise I was to create a experience within my own system that through a few different demo’s in 2024/2025 left me quite impressed.

The New Amp’ and Speakers are used in comparison to my main system and the result is I am won over.

This same experience led to thinking about changes that can be made within the main system and if all goes to plan these will be in place prior to 2026.

I strongly suggest looking at a modern driver design Coaxial Speaker with a Driver offering over 100dB for the upper frequency and close to 100dB for the lower frequencies.

 Note: When adopting High Efficiency Speakers, noise within a system will be very speedily discovered, resulting in additional work within the system. For myself, I can happily push volume above normal listening and experience a quiet background. It might be worth cranking up the volume to learn what is revealed as background noise in the owned system to see how HE Speakers will integrate initially.