You have a very nice looking system. I am a fan of Onken bass reflex loading too. I have a system with twin 12" woofers in an Onken cabinet. My midrange horn is NOT built into the cabinet like yours, and instead, sits on top of the Onken cabinet. It is raise up so that a bullet tweeter, like yours can sit under the horn. My midrange driver and horn are Western electric models -- 713b compression driver and KS12025 horn. Amazingly, I know another person with a system with Onken cabinet and the same 713b/KS12025 horn combination. This is particularly surprising given how extremely rare are the 713b drivers (very hard to find them in mached pairs for stereo).
These highly efficient systems do sound great with low-powered tube amps. I've run mine with pushpull 45, parallel single-ended 2a3 and a pushpull 349 amp. My favorite amp is the pushpull 349 amp that puts out a whopping 5 watts per channel.