Mass loading Tannoy Definition DC10a speaker.

I would use sand not lead. Are you attempting to tighten up the bass?

Thank you for your input. Possibly tighten the bass and make it deeper.  In general I am trying to get it to the next level, if applicable. I am happy with the way it sounds now. 

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 :) 

My Cornwall 4's are easy to mass load from the bottom because of the 3 bass ports at the bottom facing forward. I used lead weights and found that at some point weight became counter-productive and actually boosted and distorted treble. The sweet spot was about 18 lbs of lead per box. More weight stiffened the walls of the box too much and perhaps contradicted some of the resonance designed into the speakers in the first place.