They were made to be stacked . I recently sold mine to a buddy who has them stacked just like I did for years.
question for SLP05 owners:
Back when the SLP05 first came out, the (separate) power supply was designed so that the actual preamp part of the SLP 05could sit on top of the power supply. I was wondering if any owner/operators actually went that route, as I am thinking of re-configuring my listening room so that I can use shorter speaker cables, and placing the preamp on its power supply might make that transition easier. The reason I am asking, however, is that I have noted that the power supply gets really hot after a couple of hours or so of listening, and I am not feeling real great about having the preamp right on top of it. Therefore I was just wondering what the experiences of others has been.