The tubes will see a higher impedance on the primary if you use the 8 ohm tap on the secondary. With the 8 ohm tap I'd expect lower overall distortion (maybe) but less overall power available from the amp.
If I am not mistaken, you have it backwards.
Whatever load is connected to the secondary is "reflected" onto the primary side by the impedance ratio between the primary and the secondary.
Suppose the transformer has an 8000 Ohms: 8 Ohms impedance ratio.
If the a 6 Ohm load is connected on the
8-Ohm tap, the "reflected" impedance on the primary side is:
6 Ohms * (8000 Ohms)/(8 Ohms) = 6000 Ohms.
If, on the other hand, a 6 Ohm load is connected on the
4-Ohm tap, the "reflected" impedance on the primary side is:
6 Ohms * (8000 Ohms)/(4 Ohms) = 12000 Ohms.
Matt