I would expect that the 8 ohm tap supplies the highest swing, the 6 ohm less, and the 4 ohm least (that would be consistent with transformer winding ratios for the impedance matching. If you use the 4 ohm tap on an 8 ohm nominal speaker, depending on the speaker efficiency and your sound level requirements, you may find need to advance the (volume) level to the point that you may drive the amp output stage into voltage limitation (clipping). Conversely, if you use the 8 ohm tap on a speaker that exhibits considerably lower impedance over much of its range, you could encounter current limitation (again depending on the sound pressure levels you desire). I would suggest that youmight try each tap, starting with the 8 ohm and starting with your level control down, and use the one that gives adequately loud listening level with a reasonable position of the level control; that is, not just cracked or almost full up. I'm guessing you'd end up with 8 or 6. If you perceive a sound difference, you're on your own. :-)
