... speaker stands other than concrete blocks...

Hello to all...

Please advise if you have any experience with speaker stands other than concrete blocks for wide, heavy, large bookshelf speakers: mine are JBL L25 PRIMAs (approx. 19" wide x 16" high x 15" deep); very funky cabinets, 35 lb/each. Not mirror image driver orientation. 
Have been investigating on the internet - and I believe most top plates (6" x 6-9") are far too small to adequately balance a heavy, wider-than-most speaker on top.

OMG, I had those. Technically L26, same exact speaker only in a yellow composite cabinet that due to its monstrous appearance was heavily discounted making it perfect for my meager 1975 McDonald's burger flipper budget. 

Cinder block covered in the right fabric is a pretty decent DIY. Those speakers are big and heavy enough to benefit from a large massive stand. Probably not a lot on the market. A good DIY would be 4 pavers for mass, laminate MDF to kill vibration and provide an attachment for one or two columns between each pair. Sand and paint.

You could also make them entirely from cast concrete. Dirt cheap, most of the "cost" is in the time and materials needed to make the mold in an attractive shape. My turntable rack for example is cast concrete. The columns are polished ABS filled with concrete. It all bolts together. Same general idea would work just fine for your stands.  https://systems.audiogon.com/systems/8367
Sound Anchors will make you a stand of any dimensions you request. Their stuff is great.

...thought I'd do the standard 7-5/8" high x 16" wide blocks (3 high, maybe with some sorbathane pucks in between), cover them in FlexSeal (liquid rubber) and top them with an iso-stand 430 (wide arrangement). Stand would weigh 105 lb +...

Also - a completely different ?: Since you once had 'um - could you suggests an entry level (as cheap as possible) integrated amp you think might couple nicely with the JBLs?