Will the placement of casters on speaker stands degrade the performance

Use whatever you need to get placement (take all the time you need), you can then move to superior footings?
Like a trolley to move furniture, just don't leave it under there after it's in it's new home.

And yeah, there's no surprise from me, do try springs. Boing boing!

Will you care? none of us can say.  Will it technically degrade the sound? Absolutely.  It will allow more motion of the entire speaker assembly, which modulates the transducer.  An ideal speaker would have zero motion.  Castors are wheels. With bearings.  Which move.

Consider that some people go to the effort of placing speakers on spikes; and some manufacturers suggest that you fill the stands with shot.

In fact, if it were feasible, the way to overcome the castor problem would be with strategically placed mass so that the (lack of) resistance of the floor thorough the castor would not matter.

But if you need to move them regularly (say, out of the way for use of a living room and out for listening), then the benefit may greatly outweigh the loss.
Happy 4th.

OP,
I suspect that you know where the 1.7s were positioned to a very high degree.  Having said that I would put the 3.7i's in the same position as a starting point.  

Go out and buy a 4'x8' sheet of masonite less than $20:
https://www.homedepot.com/p/Hardboard-Tempered-Panel-Common-3-16-in-x-4-ft-x-8-ft-Actual-0-175-in-x-...
Have them cut it in half.

Now you have two 4'x4' sheets to center under the speakers in the aforementioned placement.  Speakers should move very easily as masonite is pretty slick.  You won't have any height problems either.

Just a different take on your issue.

Regards,
barts

Why not use the castors to move them into place then switch over to solid footing once you obtain desired sound stage?  A dealer here strongly recommend master setting.  I wonder how many in this group have set their speakers this way.