Some options:
Sugden A21 SE Signature. Standard rack width, height about 4 in. Depth about 12 in (as with any class A, you will need air around it. Integrated.
Musical Fidelity A1 (new version) rack width, height 3 in. Integrated.
Schiit Audio Aegir 2: 30W @ 8R stereo, 100W @ 8R mono. Needs a preamp (which they also make several). Compact, 9x13x3 7/8"