I don't think changing the amp will help you. I have head those speakers at a high end dealer powered by exotic amplification.
For small speakers they are very good. However they still sound like small speakers. There was a slight "brittleness" to the sound. I associate this with a deficiency on the upper bass power response. This happens with small speakers.
The reason is the transition frequency between the half space/full space transition. This is related to the baffle width of the speaker. The smaller the speaker the higher the transition frequency. This requires baffle step compensation. In a passive speaker this is done by dropping the impedance below the transition frequency. However their limits as to how far you can drop the impedance, and small drivers are limited in cone excursion and power handling. So with small speakers like that you can pretty much never get total baffle step compensation, in a passive design.
In your speakers the impedance drops to four ohms at 200 Hz.
Now look at the FR, which for a small speaker is very good.
The red trace is your speakers. The blue the famous small BBC LS3/5A monitors. You can see the response of both are very similar. But in both you can see a slight fall in response below 600 Hz. This is right where you would expect the transition frequency to be in small speakers like that. So the loss in output is about 4db below transition. Now that is small, but in a region where the ear is very sensitive, so it will be audible.
In summary, it is small speakers that are holding you back and not the power amp.
A better sub will help mask it, but a sub does not produce output at the critical frequencies where your problem is occurring. So you solution is bigger speakers and not a different amp. Small speakers always are a compromise. The old adage applies: - "Do speakers have to be large? No but it really helps!"