Marantz HD 770 speakers bass drivers

You're going to have to do a little research. My guess is that Marantz had these speakers build for them by a speaker manufactor. Wouldn't even be surprised if your HD770 was a clone of one of the manufacters speakers. Find out who build these speakers for Marantz, and what drivers they were orginally using. Now one thing to consider, is whether you can find some orginal bass drivers in good condition. Maybe yes, maybe no.
If I was you, I would be inclined to keep things as it, especially since the speakers seems to be giving you "impressive" performance.

I own 2 pairs of the HD-880's which are the same as the HD-770 except they are towers instead of the floor design of the 770.

The original foam surrounds would rot over time and many repair shops would replace the entire woofer instead of fixing the surrounds. One of my pairs of HD-880's had replacements and they were defn inferior to the originals. Originals had a very heavy magnet and very good quality. Could not believe the sound difference going back to original!

It really comes down to money. The originals may sound better than what you have and it makes the speakers more valuable especially if the cabs are in good to VG shape.

I found my replacements on ebay but it is hit or miss on price. Personally i would go back to originals.

The HD line up IMO is so under estimated and not known about by many audiophiles. These speakers sound great and even better matched to the vintage 22 and 23 series receivers from the same era.

I have run them with my 4400, 2385, 2325, 2285B and 2275. What incredible sound!