How do you feel about a wheeled dolly permanently under large floorstanding speakers?

You are making a simple problem into a complex one by thinking of a mobile platform. I strongly suggest you do not use a mobile platform, but simply put them on casters. I have used a few speaker systems on casters and they are wonderful for ease of positioning. 

People may insist that spikes are necessary to elicit the best performance. While that may be true, I have not found it to be the case. I have used many speakers that have spikes, and frankly, the speakers that have had casters performed better with casters, largely due to the elevation of the speaker raising the sound stage. In addition, there are literally dozens of potential advancements to systems, and almost never are all of them employed. Imo, spikes are among the least important of the changes that can be made to audio systems. While they can confer a change, it pales in comparison to what is done with power and signal paths. (I am not interested in arguing my advice with anyone here. If you disagree, great, do things your way.) 

A tangential point; most people randomly pick a position for a speaker system. With casters, you can experiment MUCH more with it, and most likely find a far better result. I would never put a big speaker on a separate, mobile stand, as it is far less stable, too. Talk about introducing unwanted vibrations! Casters would be far better in that regard. 

In real world system building you make the changes necessary to live with the system - or else you won't use the system as much, and or have trouble with the spouse. 

caper, yes, absolutely, casters on the bottom of the speakers in place of the spikes. It works beautifully!

Note, it will probably yield a superior result to spikes; as I said, the (often) additional elevation of the soundstage is considered by the ear to be advantageous. You don’t even need locking casters - unless you have a downhill slope to your floor! ;)

Here’s another reason to consider casters; you can probably alter the front baffle slope to improve the speaker. The Vapor Audio Joule White speakers I have do not have perfectly even height of the casters - I intentionally altered their height. The rear casters have two additional washers, which lift the rear of the speaker slightly, adding a hair more forward baffle slope, and imo give the speaker even better sound. Of course, it’s completely adjustable, as one can add or remove washers to suit. Imo, that is a far more important adjustment than whether the speaker sits on a spike or on casters.

With casters you can quickly, efficiently, check whatever placement you wish, and I encourage you to not waste time. You do not need days or weeks to move the speakers to your preferred location. Move them several times in a day if you wish. You will dial in a much preferred result quickly. There is no advantage to wasting your life on it. If you want, change it back, etc. Take a piece of tape and make a mark or two for preferred locations. It’s simple.

Absolutely, use a lock washer for the caster to cinch down. Select a robust caster, and don’t be chintzy about it. On wood floors be careful, as the weight of the speaker on the caster as it moves can put a running compression line in the wood! You do not want a thin wheeled caster, but one with more width to distribute the pressure so you won’t get damage. Note casters that can handle higher weight, and still have width to be ok on delicate floors. There, I just saved your flooring. :)

bob540, I did not seek a specialty audio product, just as I did not seek a high end rack for my gear. I have made choices where to allocate the funds, and I prefer to allocate them in the actual power and audio chain. Note, however, that I am doing digital source only, and would likely have a different perspective if I did analogue. 

I say this to give you some perspective on my advice. I use rubber hockey pucks under speakers occasionally for the same reason; cheap, effective, and allows quick changes which I do continuously. Were the speakers anchored to one spot always, I would probably use spikes. But, it's not practical for moving about. 

I would look for spikes used for large furniture, such as big cabinets. Maybe check with a furniture store, or an office store. It's pretty easy to get larger, black ones with wider/double wheels. Mine have the twin wheels per caster with a gap between (like dual wheels on a pickup truck), and they are more than 1" wide so as to roll on the carpet easier. I think I got the casters I put on the Vapor Audio Joule White speakers at Home Depot. Tried to get better quality ones among the selections. They came with longer threaded posts than I was searching for; no problem, I added washers to close the gap, and frankly, the speakers are better sounding with the extra inch or so of elevation as it lifted the soundstage very nicely. 

For the large, 175 pound amps I put on modified furniture dollies, they had solid hard rubber casters built in perhaps close to 1" thick. I can push the amps around pretty easily, even though the flooring is thick pad with thick berber carpeting. So, 60 pounds should be quite easy with wide casters, no issues. Except, watch out if the speakers are taller and thinner! You don't want them tipping when moving. Sitting still, no problem, but have a good handle on them while moving, because if they are narrow, they will tilt most likely as you move them on carpeting! 

Take a spike along, and match up the thread. Take your pick on size of caster, and how long the threaded post to insert. There are only so many sizes of posts with specific wheels, so you have to be somewhat flexible. It's not hard.