You won't hurt the speakers by doing it. Depending on whether you hook them up in series or in parallel, you might stress the amp since parallel will give a lower impedance, which not all amps can handle, but if you start at sensible volumes and listen for distortion or signs of distress, and feel the amp to see how hot it is, you shouldn't have problems with any decent amp if you are just trying it out - long term might be different. I suspect that you will have more, but not neccessarily better, sound. But the whole stacking thing, which came about with stacked Original Advents, seems to have been genuinely worthwhile.
Here is my experience: The A 25s are remarkably good sounding speakers for their price, size, and age. I listened to them when they were current and have had several pairs over the decades, and my experience of them was more or less the same. The A 35s I picked up twenty years or so ago didn't impress me much, ditto the A 50s and A 10s, though the A 50s were certainly fine - the A 10s were a big disappointment. My suspicion is that there were aging cap problems with them. Why the A 25s didn't suffer the same problem I don't know, though I suspect I may have been hearing what I expected to hear. At any rate, I'd recap the A 35s, and maybe the A 25s before solidifying any opinion about stacking.
