In general, we design and build both solid state and hybrid tube equipment because some potential clients will have nothing to do with solid state equipment and others don't want the task of dealing with vacuum tubes.
Of more interest is that as our design skills improve, the sonic signature of all our equipment, tube, hybrid, and solid state, are converging. We would suggest that if the designs were actually perfect, the sounds would be identical.
We are not there yet, but as I often go from design type to another in final testing in my reference system of all our new equipment we ship, it is really tough to say which are the best. They are all really good sounding as our clients know.
The balanced mono SET 600 and 850R designs are a bit different. They have musical advantages over their stereo counterparts. Running full differential mode from source to speakers is an advantage as any very low level distortion and noise is eliminated and there is even more isolation between channels. To my ears, this provides an even better three dimensional sound stage and transparency. Of course this comes at about twice the cost.
The SET 400 compared to a Fet Valve 400R? The SET solid state design is to me a "high energy" design, capable of resolving the tiny microdynamics and energy of a string instrument amazingly well without grain, glare, or any unnatural brightness. It has an enjoyable "wow factory" from top to bottom and is definitely the choice to perk up the bottom end of any speaker. It probably is not the amplifier for speakers running towards the "detailed" or slightly hot side. The Fet Valve amps are a bit more relaxed with very engaging and a very slightly warm presentation. The Fet Valve amps do have a vacuum tube front end after all.
In any event, a pair of Vision SET 600 mono amps will outperform our stereo Fet Valve and SET amps, while the DVA Fet Valve 850 mono amps, which are crazy powerful, do best of all.
All our amplifiers are happy into 4 ohm loads and have no issues with 2 ohm loads either, up to the max recommended speaker fuse ratings. They are just as musical at very low listening levels as at high, but because they don't stress out at high levels, you might want to crank up that volume control to better match live performance sound levels.
Remember too that all new AVA equipment comes with a 30 day satisfaction guarantee so it is hard for you to go wrong choosing our audio equipment.
Frank