In the last couple years there have been about 20 posted to Fleabay, of which 18 were non functional.

The 771A sounds better, but blows up worse.

The B doesn't sound good and still blows up.

OTOH the rack mount Altec biamps are nice.

Vintage Altec 1609A Biamplifier 150W 50/60Hz USA - eBay (item 320545003392 end time Jun-14-10 12:25:55 PDT)