A quick look through Parts Express with enough sort criteria gives a very promising concept: the Dayton Audio CX120-8 in a .2 Cu.Ft. box with a 1.5″D x 3.75″L port will tune down at about 70 Hz with an F3 at about 63 Hz. Pretty impressive for a 4″ driver. And with the coaxial tweeter, you keep the smaller space and higher mounting of the single driver. looking at the FRD graphs, you could get away with a single order crossover (just a single coil and a capacitor) and most likely be happy. You’d probably want to pad the tweeter down a touch.
Since your 300B is a low power amp, i wouldn’t be worried about much of the things most others might bring up, like port velocity, etc. it is only 8 watts. But that should get you up to somewhere around 93-95 dB in your near-field setup. Plenty loud enough.
Anyway, that’s my initial thoughts. Anyone else is welcome to jump in.
BTW, who makes your 300B? I’ve heard there’s an emerging market for them. I’ve been waiting for Mark at Blueglow Electronics to get back to his design.