Heavy difficult to move speakers- how do you deal?

I am an advocate of alternate positions, alternate toe-ins, especially related to Imaging for one or two listeners.

I move mine frequently

Furniture Type Dual Wheel Caster's Axels are firm, single wheel axels, even high quality, wobble. 

3 (not 4) Casters. will always be steady, no shims needed anywhere you move them. More downward weight (+33%) per caster.

An apron/skirt (my current speakers shown below) concealing the casters automatically prevents tipping.

Rear Anti-Tip blocks. 2 casters up front, 1 rear center caster, no apron/skirt: you need to prevent tipping.

Put 'anti-tip' blocks in the rear corners, just a little shorter in height than the casters, they only touch the floor if the speaker starts to tip. 

Lastly, tilt: aim the tweeters at seated ear height (see the unfinished pine 2x4 above my front casters, lifts the front 1-1/2". The upward tilt also alters the angle of reflection off the floor and ceiling

+1 to sliders

OP,

I would imagine it would be possible to put castors as there is a drilled out steel plate forming its bass for the steel points. , But to take such a spectacular speaker and then significantly compromise the sound quality  with castors would be a real shame. 

Another vote for future sliders.  I have them under my speakers and my amps.

don't put the spikes and protectors in place until you have your speakers in place.....then tilt the speakers for inserting the spikes