When you say your father had the woofers "replaced" please tell me that he had the original woofers re foamed & did not replace the factory woofers with aftermarket parts.
As with all vintage gear value is what a collector will pay on any given day,the HD 770's are pretty rare & are sought after by Marantz collectors,value is also dependant on condition,most non audio collectors/gear heads have a different idea of what the word "mint" really means,using the Audiogon grading scale for used gear mint is only for new & unopened gear,this is the scale that most collectors use as their bible.
The rating scale is from 1 to 10 with a 10 rating being new or unopened,a 9 rating is for the age of the gear,all vintage gear automatically starts off at a 9 unless its unopened,an 8 rating is if the speaker has ever been used at all even if there is no wear & tear & they look new, a 7 rating is if there is one imperfection anywhere at all on the cabinet/grill/drivers/front/top/bottom/rear/binding posts ect,any serious damage like dented or damaged corners or gouges in the veneer drop the rating very quickly,water stains are a killer also as are ciggerette burns.
As you can see what was thought to be a "mint pair of speakers deserving a rating of 10 quickly becomes a 4 or 5 rated speaker,every last nick on the cabinets or snag on the speaker grill effects price,no matter how small the damage is,having the factory packing/box'es & manual are a huge plus in resale price.
With all of this being said the value of them can easily hit over $1,000 to the right person if the speakers are in perfect condition or they can be worth as little as $50 depending on condition.
If you can send me a bunch of well light hi res pics from every possible angle i can give you a ball park figure but i'd also need to know what really happened to the woofers & if the originals are still intact.
