What kind of speakers do I need?

Several of the amplifiers I'm offering are single-ended designs that use a single tube for each output channel, and these generally have relatively low power output, anywhere from around 3 to 7 watts or so. This means you should have somewhat sensitive speakers if you expect a loud listening volume or have a large room to fill with sound. Speaker sensitivity is measured in dB SPL (sound pressure level). Because of the popularity of solid state amplifiers and the ease and low cost to have high powered output from transistors, many consumer level speakers today are lower in efficiency. A tube amplifier can still work fine with lower sensitivity levels, but I recommend speakers that are at least 90 dB SPL for best performance.

I also offer higher powered push-pull amplifiers that have a maximum of 15-20 watts per channel, and this is for people who have lower sensitivity speakers and prefer relatively loud listening volumes. In most cases you shouldn't need anywhere near 20 watts for typical listening volumes.

Speakers are one of the most important parts of your sound system, and of course it isn't reasonable to expect good sound from a great amplifier with low quality speakers. Frequency response especially at the low end is difficult to produce without physically large speakers. While I try to have my amplifiers reproduce high quality sound across a typically accepted audible frequency range of 20Hz to 20kHz, most speakers cannot reproduce sound down to 20Hz, and almost no music would ever go that low. If your speakers can do well from around 40-60Hz and up, you should still have a pretty good bass response. Likewise, if you've gone to too many rock concerts or bad marching bands, then your ears may not be able to hear really high frequencies approaching 20 kHz. But again, most music will not be in these extreme values very low and very high.

I design my amps typically for 8 ohm impedance, since this is the most common speaker impedance used for home stereo use. But some of the amps have output transformers that can be wired for other impedance such as 4 ohm speakers. And remember that speaker impedance is labeled nominally, but actually varies significantly over the audible frequency range. It might be 8 ohms at one frequency and 6 or 10 ohms at another, and so on. I have a pair of speakers in my home now that are 12 ohms nominal, and use them with an amplifier designed for 8 ohm impedance and it works very well.