I'm looking into building my own amp for a home subwoofer. I havn't been able to find any plans for one on the net. I'd rather make a tube amp (for asthetic reasons only). I'm only doing it as a hobby, I'm sure a powered sub pkg is cheaper.
Unless you have a very efficient subwoofer system in mind you'll need substantial power from any amplifier meant specifically for low frequency operation. It's not uncommon to see recommendations of between two to five times as much RMS wattage in the subwoofer amp as what will be used for the main speakers. That's a basic problem with tube amps. Firstly, high power is not that common in tube amps due to the size, weight, complexity and cost. Secondly to find high power in a diy tube amplifier is somewhat against what is being sold right now. The market in DIY tubes is very much in the low power SET amps which run tubes such as a 300B.
If you want to explore tube amplifiers, just place "diy vacuum tube amplifiers" in a search engine to find the shops that can help. But don't expect to find tube amps designed for subwoofer use.
If you know how to read and construct from a schematic you can access plans for classic amplifiers like the McIntosh MC275 on line. The cost of constructing such an amplifier from scratch is about like constructing a 1967 Mustang from parts.