That gap looks quite small to me, and is probably constricting the output from the TL. You're correct, to add spacers you will need to lay the speakers on their side and then the plinths unbolt from underneath. Adding spacers is as simple as finding something to slip over the thread of the bolt, between the cabinet and the plinth. Washers, either metal, plastic or nylon, would do the job if they are a reasonably snug fit.
I wouldn't say that the Howard was bright, no. The smaller Harlech (which I owned - the Howards I refer to belong to my father) could be quite "forward" in the upper-midrange and extended in the highs - a lot of people don't like this style of presentation, finding it fatiguing over time. The Howards are smoother I find, although accompanying amplification will also have an influence here. I'm not familiar with your Electrocompaniet so can't comment here.
The later S3 Castle models with the carbon fibre mid-bass have a darker tonality. I had the Conway 3 and couldn't adjust to the ported bass and change in tone over the Harlechs; many would probably prefer it overall.