pro's and con's of a 4 ohm speaker?

Again, it all depends on the interaction between the speaker and amp, the impedance , IN ITSELF, does not make the speaker sound better or worse. If your amp can handle the load fine, if not update your fire insurance.

In general, amplifiers are happier into a high impedance load, as long as they aren't driven into clipping. So assuming all else is equal (which of course it never is), I'd suggest 8 ohms if there is no danger of clipping and 4 ohms if there is.

Now all else is never equal, and apparently you've already found speakers you really like, and unless their impedance curve is really funky they'll work fine with both your amps. Having "speakers you really like" outweighs any theoretical/speculative "amps are happier with higher impedance unless blah blah". So go for it!

Cwaz - Power will be inversely proportional to load (assuming good power supply) while perceived loudness goes in (1/3.5) power of the amplifier output power ratios.

In you case: Loudness = (8/4)^(1/3.5)= 1.22 (22% louder with 4 ohm speaker)

8 ohm speaker will also double your damping factor (damping of the speaker).

8 ohm speaker will also double your damping factor

Although that will not be true in the case of a tube amp, assuming the 8 ohm speaker would be connected to the 8 ohm tap, and the 4 ohm speaker would be connected to the 4 ohm tap. In those cases damping factor would be the same.

Not that it matters in many cases. Damping factor is mainly significant when either:

1)A tube amp (or other amp with high output impedance/low damping factor) is used with speakers whose impedance varies widely with frequency. In which case higher damping factor may or may not be better, depending on the particular speaker. Or,

2)Woofer damping and control is particularly important to the particular speaker, and the damping factors of the amps being compared are not especially high.

Best regards,
-- Al

Al, I always forget about tube amps since the last one I had was EL34 100W guitar amp I built about 40 years ago.

As I understand it combined plate impedance gets divided by speaker impedance to get transformer turn ratio. If plate impedance is 1k then transformer ratio for 4 ohm speaker will be 250. There might be negative feedback involved but it will lower impedance in the same ratio thus 4 ohm tab impedance will be always half of 8 ohm tab impedance.

Is there any benefit of using lower impedance speakers with tube amps? I remember 16 ohm speakers and heard of 32 ohm designs. Didn't low impedance in speakers come with SS amps?