Many thanks for your encouragement!
The washing machine is now running fine with about 1/4" of washers as shims/spacers between the side panels and the front edge of the top panel.
Unfortunately, as a result of raising up the top panel, the holes for the two screws for securing the front panel near the bases of the side panels don’t line up now - and consequently the front panel is hanging (and hinging) from the spring clips at its top edge.
Also, it seems strange to me that there is room whatsoever for a rubber seal between the tub and the top panel (and there never has been since I acquired the washing machine, second hand).
Is it feasible that some other repair done before I acquired the washing machine resulted in a higher position of the tub, or even that the original tub was replaced with a slightly taller one, or something else like that?