Speaker for 300b amp?

This seems a little backwards to me. Cart before the horse thing. Speakers have by far, the most effect in the reproduction chain.

Makes far more sense to buy speakers you love and then search for suitable amplification. 

I am using Klipsch Forte IV speakers with an Audio Note Vindicator amp.  The amp uses two 2A3 SET tubes rated at 3.5 w/ch which is plenty for the 99 dB sensitive Fortes in my medium sized room.  I suspect that they would do fine in a larger space.  I doubt that I ever power them at as much as a watt.  Taste in speakers being highly subjective, Klipsch may or may not be your cup of tea but at least there is the advantage of being able to audition them to see (hear) if they are for you. 

You have some good recommendations above.  I have two 300B amps, both running WE tubes, and have been playing around with speaker combinations with my Cary integrated amp for 15 years.  I like horns but recently discovered Omega Audio speakers made in Connecticut by Louis Chochos.  I was having trouble finding a good fit for my Tool Shed Transcendence (8 wpc) in a smaller room (14’x15’) and his Super Alnico Monitor really did the trick.  These speakers have no crossover which was new for me but the range on his drivers is unbelievable for a stand mount speaker.  He’s easy to talk to – I recommend calling him and having a conversation regarding your setup and needs.   He also has floor standers if bookshelves aren’t your cup of tea.

The other speakers that I’m in love with are my Volti Rivals which I have in a bigger room.    These horn-based speakers are made by Greg Roberts in Tennessee.  Like Louis, Greg is easy to talk to.  I visited his shop when I traded him my 40 year old Klipsch Belles.  Spent the night listening to all sorts of music with him and then camped out overnight in his veneer room.  Six weeks later I was picking up a beautiful pair of Red Gum veneer Rivals.  His line of speakers are quite a bit more expensive then the Omegas but boy do they play... like you’re in a live music setting...and they resolve.  If you are looking at any of the Klipsch Heritage line you really need to speak with Greg and schedule a demo.   His YouTube videos are a hoot.

The other option I might recommend is finding an older pair of Klipsch Heresy’s, Cornwall’s, or Lascala’s and doing a little crossover and driver updating.   Greg sells DIY kits for the LaScala’s, Belle’s and KHorn and I think he may even take on an upgrade project himself if you can convince him to re-veneer the cabinets.  I have 2 pairs of Heresy’s (1&II) that I’ve worked over myself (Crites parts) and they sound great with my Cary 300B integrated (15 wpc).  I put them on stands to get them off the floor which I think improves their imaging.

Hope this helps just a little bit.  Good luck and happy hunting!

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