If you must beef up your floor you could remove the sheetrock below and put new joists between the existing, spanning from bearing wall to bearing wall. This would make for 8" o.c. which should make for great rigidity.
Also, consider that I have a 130-140 lb turntable (Teres) which sits atop a custom mass loaded stand which weighs another 100 lbs or so. This is sitting on a floating wooden floor using 10" joists @ 16" o.c. with a single sheet of 3/4" plywood flooring. I can jump in front of my TT while it is playing with no detrimental action to the TT's performance. However, I do have it braced from beneath. But it is not extreme bracing. Only a few uprights were needed.
Just something to consider
Also, consider that I have a 130-140 lb turntable (Teres) which sits atop a custom mass loaded stand which weighs another 100 lbs or so. This is sitting on a floating wooden floor using 10" joists @ 16" o.c. with a single sheet of 3/4" plywood flooring. I can jump in front of my TT while it is playing with no detrimental action to the TT's performance. However, I do have it braced from beneath. But it is not extreme bracing. Only a few uprights were needed.
Just something to consider