I'd like a high-quality A-B-C switch to compare 3 amplifiers. I'd like the transition to be (almost) inaudible. Obviously, it would have to switch the line-level inputs and the speaker outputs. What's available off-the-shelf (I already have too many projects to build one)?

I'm also looking for a high-quality A-B-C switch to compare 3 speakers. There are a few choices on the market. What are everyone's preferences?