The OP asked Alberto Guerra (AGD) how his amplifiers sounds, well I own the Vivace and I can say they sound "wonderful". They bring a very live feel to the music. They are an "end game" amplifier in my opinion. I went from a pair of tube mono blocks (Consonance Cyber 800SE) that I loved, to trying a pair of Electrocompaniet AW 400 solid state mono block amplifiers in my search for a little more bottom end without loosing the tube magic. I did quickly go back to my tube mono blocks not being happy with the mid and upper frequency ranges that solid state brought. More bass was not worth loosing the "tube magic" of my mono blocks. For a short moment in time I had considered going with a Pass Labs Class A design but due to the heat, expense of running them and their weight I decided to explore Class D now that it was coming of age. After reading numerous reviews on the many quality Class D designs I came across a number of glowing reviews and testimonials on this new line of unique amplifiers designed and built by AGD. The two amplifiers I had my eye on certainly were unique looking so I reached out to Alberto and had a number of long conversations with him. Finally I decided to audition the Vivace and he shipped it out to my house on the east coast for an audition. This was the quickest audition I ever had in my life. Within the period of one song, before it ended, I knew I had a winner on my hands. I had my tube mono blocks off to the side because I felt I would be switching them back and forth listening for improvements. Well within one day I knew the switching back and forth was not necessary. I just carried each one up to the third floor and that is were they are now. Never once did I switch them out, it was so apparent these were game changers. The Vivace just blew both of my previous amplifiers out of the water along with any thoughts of a Class A amplifier.
The Vivace are quick, detailed, full sounding amplifiers that can awaken any speaker (mine being a pair of KEF Reference 3). I was in disbelief. Every aspect of my speakers sound improved radically. I auditioned them for a week straight listening for hours at a time trying to find fault. I invited my audio friends over to listen and they all were blown away at the improvement and the look of the glowing acrylic top and the tube. Yes it may appear a bit too much when you see them not lit up and the tube glowing a reddish orange, but once they are on and the room is darkened they are gorgeous and not over the top at all. Luckily they sound even better than they look.
The sound stage is expansive beating my tube mono blocks ability to through a wonderful sound stage. Not night and day but it is noticeably deeper and taller. The positioning of musicians within this sound stage creates a very real experience. The musicians hold their places and when two musicians share a mic or are standing close on the stage you can "see" and hear the smallest gap between them as they stand or sit there and perform for you. It can be a very intimate experience at times how real it sounds in terms of placement of the music in the room. Obviously this happens on the better recordings and as everyone knows it is very recording dependent. The low end to mid has much more authority and detail than even the Electrocompaniet provided with it's 400 watts per channel. The Vivace does not come anywhere near that power but I have never felt it lacking in power to drive my speakers and I doubt it would struggle with the most power hungry of speakers out there. The mid range is beautiful sounding, vocals stand out and are articulate. Instruments have so much more vibrant detail to them. Up right bass and violins have the sound of wood and you can almost feel the bow drawing across the strings. Eric Dolphy's "Hat and Beard" from Out to Lunch is an incredible song to listen to which highlights this amplifiers ability at detail retrieval and ability to create an incredible sound stage. Cyrus Chestnut's "Cornbread Pudding" showcases the amplifiers ability to reproduce piano and upright bass. If you are familiar with these two songs then you know that this amplifier is up to almost any task since it handles them effortlessly in terms of retrieval of the smallest detail that brings these pieces to such life that you feel the performers right there in the room and can hear the slightest and most delicate of details.
These Class D amplifiers pair up beautifully with my tube preamplifier (Backert Labs Rhumba Extreme) which I am sure is helping with the sound stage and detail retrieval, but previous amplifiers could not bring so much life to my music so yes they pair well with tubes but I am sure will work with any quality preamplifier. As much as I love my preamplifier, based on the quality of this amplifier I would to hear how it pairs with the AGD Andante preamplifier/steaming DAC that I have read about. Again another costly piece of equipment and well above my price point so I am very happy with what I have but it would be interesting to hear how the pairing would sound.
Concerning the glass piece that extends out of the acrylic top, well at first I was a little taken back by it, but it truly is a piece of art and nothing can compare to how beautiful this amplifier looks at night in a room where the light level is low. In addition, as previously mentioned, it is extremely easy to pull the glass tube out and switch it with another module or if I were to transport the amplifier pull the tube and box it. The packaging the amplifier came in is another story and I am sure helped to raise the price but that is the point, everything about this amplifier is high end from the packaging to the design to the sound and especially to the care and support Alberto himself gives you. What you are not getting is a card board box sent to your house with the corners bashed in and lots of Styrofoam and peanuts all over the floor and if an issue arises you are not put on holding waiting for someone to answer and resolve your problem.
Yes this is a very expensive amplifier but far from the most expensive. I have been in this hobby for years slowly growing my system and the one thing I am sure everyone knows as you spend more money on each piece within your system the better your system sounds (sure some mistakes are made on the way). You cannot expect a $300 dollar amplifier to sound as good as a $3000 dollar one and so on as you go up. Yes diminishing returns but from my experience going from a $4000 dollar amplifier, which was approximately the list cost on my tube mono blocks to the Vivace was far from a small upgrade. It was actually one of the most noticeable upgrades I have made. The other was going from a Conrad Johnson Preamplifier to the Backert Labs. After years and years of upgrading and trying to reach the the best system that I could afford I am very glad I discovered this amplifier and in my opinion it is worth every penny I scrapped together to afford it. I love its look and everyone should at sometime get a chance to see this amplifier in a darkened room, it is magnificent sight to behold, but more importantly everyone should get a chance to listen to it. That is really its selling point even if it is expensive and it has a faux tube in the sense that it is not a vacuum tube. I am just glad I took the chance and reached out to Alberto and had the amplifier sent to my house for that audition.