Amp Recommendation for Vienna Acoustics

Hi Timo. You have yourself a very nice speaker. It does need a amp with a grip on the lower frequencies of it. An apropiate amp and a good power cord with it will improve it a lot. A couple of more things you can do is plug the rear ports, and different speaker and ic's cables can help. The amp I use is a Belles Reference 150a version 2. I like a natural neutral sound. One that lets me hear a cool recording sound cool and a warm recording sound warm and as many shades as it can in between. I think a Halo A21 would be a good choice to, but I have not heard it at home.

I am glad you are getting familiar with the VA's. I have found on the Mozart Grands the most difficult balance is what you are dealing with. Components, mostly the amp in this case, that control the lower midrange and below, and then do not have the spectrum above that get a little thinner, harder, brighter, or edgier compared to fuller sound of the lower frequencies. But after you find that balance the quality of sound from these speakers rates extremely high. You will be rewarded for your patience and effort. There might not be a lot of other speakers that will make you as happy if like me you like to a very high degree a very natural sound and very musical and communicative. Where all the parts of the music fit so well together you forget about the component stuff.
With what you describe your amp doing now, I am not real familiar with it, you can help out your concern with a pre amp and cables to a degree. But I think the amp is the most important component for the speakers and it sounds like that amp is taking you in the right direction with your speakers.