Not all tube amps are created equal - and, for the price, many compare poorly to well-built solid-state equipment.. However, if you're willing to forego some of the nicities, there are a few kits that, thanks to a combination of low cost and high performance, can compare very well to similarly priced solid state amplifiers with the "tube sound" people love. While I would not reccomend these to most over a solid-state amplifier, if you really want a tube amp, these are hard to beat.
One of the best of these is S-5 Electronics' K12G, availible for only $156 for a full stereo kit, or $100 for a mono kit, should you desire to build one. It's a classic push-pull amplifier with a solid, low-distortion design. Though the output power is a paltry 8 watts per channel, this all-inclusive design requires no external preamp, and makes a fine match for a pair of high-efficiency speakers and an iPod.
Another useful kit, should you require a tube preamp for the vintage amplifier of your choice, is the 12AX7 preamp from Silicon Chip magazine. While I can't find where to buy the kit for the life of me, the design is both solid - featuring a low distortion 12AX7 tube with frequency response compensation - and cheap, thanks to a simple yet high-efficiency switched-mode power supply running off a 17v wall-wart.
If you want to create the preamp yourself (it's simple, and you can just use a transformer instead of the switched-mode supply if you want), take a look at Mark Houston's excellent build guide. It's a great site, and also contains information on the K12 mentioned above.