My dealer had a demo set of DC10A for years. They were loaded with lead shot immediately when received - your post just reminded me that I never actually heard them unloaded. IIRC something like 40 - 50 lbs each speaker?
I've commented before that I'm not the biggest fan of DC10A's. This particular pair was bright, a bit lean, and definitely overdamped in bass compared to other Tannoys above & below that model. I'm wondering now whether the mass loading was a contributing factor - but I'm skeptical it could have this much effect all its own. I'd say not to bother adding mass, but then again you never know how your ears & systems will react until you try somthing yourself!
More recently I acquired a set of Kensington GR in the limited "Black Ash" finish. This has the same dull-silver (pewter?) colored horn as DC10A. Nowhere will you find any documentation that states this driver is ANY different than the usual gold-throated horn (Kensington), but I swear I heard a lot of the same things here as with the DC10A. I ended up buying a floating pair of gold-horn Kensington-OW drivers, and even though they were made in China (circa 2018), they sound decisively warmer and richer than the silver-horn drivers. I much prefer them, and feel like there has GOT to be some voicing difference with those silver-horn divers, and that perhaps the "Black Ash" Kensingtons actually used surplus DC10A drivers. Also used to own older Kensington SE (gold horns), both the SE and GR gold-horn drivers sound much more alike than the silver horns.
Anyways, the silver-throated drivers did sound a lot better with a 3rd party crossover from MainlyTannoy (UK), though they're seemingly voiced brighter/leaner than the gold-horns. The gold horns also sound better with MainlyTannoy xovers - the stock xover in these Kensingtons was AWFUL. So I'd look at xover before mass loading, for sure. Food for thought, maybe :)