Presumably the woofers on the Ushers are wired in parallel, so the impedance is lower at low frequencies than in the treble region. Maybe only one woofer goes up to meet the tweeter.
At any rate, a solid state voltage-source-approximating amplifier will deliver double the wattage into a 4 ohm load as into an 8 ohm load. The speakers were designed with this expectation in mind. On the other hand, a tube amp is less likely to approximate a voltage source, and therefore less likely to deliver the increased power output into low impedances that the speaker was designed to take advantage of. So in the region where the two woofers are operating in parallel, the net output with that amplifier is less than the speaker's designer intended. The result is weak bass.
That's my guess, anyway.
Duke
dealer/manufacturer
At any rate, a solid state voltage-source-approximating amplifier will deliver double the wattage into a 4 ohm load as into an 8 ohm load. The speakers were designed with this expectation in mind. On the other hand, a tube amp is less likely to approximate a voltage source, and therefore less likely to deliver the increased power output into low impedances that the speaker was designed to take advantage of. So in the region where the two woofers are operating in parallel, the net output with that amplifier is less than the speaker's designer intended. The result is weak bass.
That's my guess, anyway.
Duke
dealer/manufacturer