Bozak concert grand speakers

Bozak Concert Grands certainly were NOT designed to be filled with sand.  I sold Bozak Concert Grand speakers back in the 70's and also have been to the Bozak factory that was on Connecticut Ave in South Norwalk, Connecticut.  Rudy Bozak showed me around the factory and I saw Concert Grands in the making.  I also saw how they made all their own raw drivers.  I repeat, they were NOT designed to be filled with sand.  The inside of the enclosures were pretty empty other than a thick type of insulation lining the interior of the boxes.

It sounds like you are not going to keep and use them, so is the question how to sell them?
They are a classic, though it would appeal to a narrower market due to size, weight and vintage. I gather that the crossovers could be improved (there was a fellow named Tobin, now passed, whose name came up frequently as the "Bozak guy"). 
You might get some traction (more Bozak enthusiasts) on some user boards with a Bozak group- e.g. Yahoo and Audio Asylum.
I don't think there are that many of these around, certainly in unmolested condition, given their price in the day, their size and the fact that your father's pair hasn't been moved since installation. I'd love to hear them! I know a guy in NY who was in a similar situation-- he hadn't set them up but was threatening to do that just at the time I moved away. Alas! :)
PS If you couldn't tell, I'm all in favor of your keeping them. 

Those are some really nice speakers if in great shape. I would love to have them but I  live to far away. There was a big openess and ease about music coming out of those speakers. 

They were great for bass, but the aluminum tweeter was rather dull sounding.  In the store, we had to tip up the treble about 3 db or so to get a little sparkle.

Beasts
my dad had the B-305 Essentially half the CG
they were not designed for sand
but were of substantial construction including a lot of batting
the crossovers will no doubt require caps
i see them for sale now and then, local pickup only
beaters around a grand
immaculate cabinets in right style, right wormhole in universe might fetch $3K
you might try Audio Classics in NY ( Vestal )
they move some used Bozak gear

lastly you could part them out, certain drivers are worth $$$$

soniclly not my cup of tea, as noted tweeters were off above about 14k
big midrange was beamy
lots of woofers to motivate...

but i I heard them properly setup and driven by two MC275 at Music Unlimited in the late 60’s
amazing in the day

sorry about your Dad