Is it "OK" to put speakers on their side?Yes of course you can do it, but think of this the prototype speaker was voiced with hours/days of xover/stuffing even driver and maybe box size changes by the manufacturer to sound that way standing up, and then put into production.
Your going to be throwing away some of his hours spent voicing it, because it will sound different to what he wanted you to hear.
Now that I've made you feel real lousy for even thinking it, don't worry about him just do it.🤔
Cheers George