I seriously doubt the issue has anything to do with your amp. In my experience, some speakers just don't open up unless you open up the volume control a bit, as you have found. I don't know exactly what this is attributed to, but I think it has to do with the driver design and mechanical stiffness. In my experience, Dynaudios can be this way. I'm not Dynaudio bashing. In fact I think their speakers are very well made and engineered, but they don't let go of the music unless you play them a little louder.....a bit too loud for my listening habits / listening distance.
You might try breaking them in more. Place them face to face with maybe just 4 inches of space between them and wire one speaker out of phase. If you can, send a mono signal to the speakers, so they're both playing the exact same signal. You should be able to turn the volume up a quite a few ticks higher than normal and not hear much more than bass sounds coming from the backs of them. You can also cover them with a blanket or something to keep their back waves from radiating too loudly and also gently plug the ports with socks (clean ones
). Play dynamic music with good bass. This should loosen up the drivers' rubber surrounds a bit and may also help break in some of the crossover components a bit also. This is pretty standard break-in procedure, especially if you are away at work during the day. I'd do this for maybe 2-3 days, assuming they've already broken in a little bit by the previous owner and you.