Hi Paul,
nice set-up, love those blue meters. I've owned a few different PMC models my self at one point - great speakers, it's been a while now.
Stillpoint Ultra 5's already recommended are costly but would be my #1 recommendation.
Are your stands filled with anything, if not should be.
An easy way to make up for that 1 1/2" would be to ad to the top of the stand and between speaker, if looking at stone standard thickness it's 3/4" and 1 1/4"solid. So you could add say 3/4" material between speaker and stand, will be more readily available and cheaper. Ensure you use blue tack dabs between the surfaces for proper contact. When it comes to that 3/4" material keep an open mind because there's numerous materials to try, wood of all sorts, man made material like corian, quartz, siltstone to just name a few, "think kitchen counter top material" Using these materials will have an effect on the sound so could get costly and time consuming but will be educational.
If I was looking at stone I would be looking at material like soapstone, limestone, corian type of material over say granite because they won't ring as granite does.
So lets assume you are placing 3/4" material between the speaker and stand now you have endless choices of different points for under the stands close to floor making sure you get something that allows you to adjust so you can get your speakers perfectly level - taking the time to do this is the icing on the cake once set-up
P.S. You would be looking for cut offs and scraps from stone fabricators, look for kitchen counter top installers, they are endless 1000's of them. Corian, find dealer locations, some corian dealer carry only 1/2" so you would be looking at them doing a edge build up. Measure the stand outside dimensions and have say 1 1/2" - 2" wide so it sits properly over the frame of the stand - they glue the material together. Preference would be 3/4" -less costly
Have fun