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.
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.