For years I have been trying to convince tube related companies to come out with a high quality five channel tube amplifier; in vain. In general, tube watts sound more powerful than solid-state watts. So five 30 watt amplifiers should fill the bill, especially if solid-state powered sub-woofers are used.
The pitch I made was to build very slender mono-block amplifiers that could be bought separately. Thus if someone only wanted a pair of 30 watt Class-A amplifiers, they would buy two. If they later want to bi-amp, they buy another pair. Or if they want a five channel amplifier, they buy five. The faceplate could be made from an anodized extrusion and it would serve to protect the output tubes, eliminating the need for a cage. A nice touch would be to bore a large hole in the faceplate, allowing a view of the tubes.
As for the decoding boxes, a market exist for a tube output based unit. Why does no one come out with it? Bytheway, a poor man's five channel system can be made from one stereo transformer coupled amplifier and five speakers and two high-wattage resistors. At first, you may not immediately like the sound, finding it too big sounding, but after a day or two, the two-speaker alternative will sound small, flat, and thin by comparison.