Henryhk,
The darTZeel improves upon the Solo's performance in the following ways:
(1) Greater tonal accuracy and density - the Solo's sound "thin" compared to the dart. Instruments and voices are more fully formed and true to live music. In this aspect the dart reminds you of great tube gear.
(2) Absence of artifacts - listening to the dart there is a clarity, transparency and absence of artifacts as compared to the Solos that allows you to listen much more deeply into the soundstage. There is greater three dimensionality to the music and greater space between instruments and vocalists and no listener fatigue. I would say that the dart is just more musical. Micro and macro detail is amazing...bass notes for example can be followed note by note all the way through the long decay. The dart gives you all of this without the warmth or colorations that most tube amps bring along with the ride.
(3) Slam - My listening room is 23'L x 17'W x 9'H so it is not a large volume. But the Temptations are not an easy load either. One of things I loved about the Solo's were the slam they delivered. I love deep, controlled, strong bass. In my room on my speakers the dart easily matches the Solo's in this regard, but with superior clarity and pitch definition.
Hope this helps. In the past whenever I tried other amplifiers I always ended up going back to the Solos because of their overall performance envelope and musicality. The other amps each did something better than the Solos, but the overall package was not sufficiently better to make me want to switch. I must admit that the ASR Emitter II Exclusive was very good and had I not auditioned the dart I might have bought it. But compared to the dart it sounds colored and a bit dark.
I strongly suggest you audition the dart, but make sure it is properly broken in before you make your final evaluation. It sounds good right out of the box, but it really blooms after about 3-4 weeks of burn in.