Tubes and light weight bass

All I could find doing a quick search was that the Ushers are 4 ohm nominal. That could translate into well below 4 ohms and could explain the weak bass and perhaps truncated highs as well. Most likely, you have a poor amp and speaker match and might have to decide which you like more and get rid of the other.

I would attach the speakers to the 8 ohm tap and then the 4 ohm taps. If the 4 ohm taps improved the sound, but still sorely lacking in bass, I would ditch the amps or the speakers, or at least consider that they aren't going to play well together.

If you cannot move your speakers nearer your front wall and/or add a subwoofer, you may want to try a more powerful tube amp whose output impedance is lower, or better yet, for good bass, a solid-state amp may be the best bet. You may not like the rest of the presentation of the SS amp, but for bass, it probably will outmuscle most tube amps.

These speakers were voiced with class A solid state amplification and are capable of amazing bass slam and authority. Suggest ditching your amps and getting an Usher 1.5 amp if you like the speakers. Keep the tube preamp. I heard them at RMAF 2006 and was extremely impressed. BTW, I am basically a tube guy.

I had the same issue with my Manley Stingray and a power cord from DCCA placed on my Cary cdp solved the problem.
Started with Don's Source pc which did a good job and now I'm using his Ref 1 on the Cary and honestly, the bass couldn't be better.
Just my 2 cents.
Good Luck!

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