Putting weights on speakers?

Of course, that is unless you're supremely lucky and you do actually find a weight changing the resonant frequencies to optimal ones (by pure good luck).

Adding 5 or 10 pounds at times incrementally  will gives you the best tuning frequencies with the load....

If you put some objects like 2-3 inches high on top of speakers that may defray the high frequencies and help clarity

best is to change the internal speaker wires and replace crappy capacitors to metalized polyproplene capacitors, you will get faster and cleaner sound with more solid highs and lows. replace the coils too if you dare.

grow a set, it will help you in the long run 

you should place weight on all equipment. one reason the VPI Brick was made 

I sugest put your speakers up side down first - then add weight on top of that new top....amazing, just amazing....the soundstage, the color of mids, the depth of bass...what can I say...just amazing
but you have to try in steps..ie. say from 20 lbs...then increase to 30 etc...till you get to the sweet spot....