They're not fragile at all, assuming they are reasonably well built and the tubes are taken out. However, they are heavy so if dropped, damage seems quite likely anyway. I've always packed them double-boxed with layers of 1 or 2 inch foam within each box. I'd probably put a third layer, straps, and handles for airline shipping... if the amp is worth much.
You could probably remove the transformers. But you are basically disassembling the amp, it's not hard if you're experienced in electronics but if you're asking the question then I'd say probably best not to... It's a lot of trouble though, any reason to not just ship it?