@tomcy6 The link that @
yyzsantabarbara shared goes some way to describing it.
It's a tough one to pin down. Let me try. Let me also say that your mileage may vary with different speakers and other system components. Thanks for also giving us a focus: i.e: vocals.
The 600a is full and rich and warm, and delivers vocals with tremendous presence and texture. It is neutral by pretty much any standard, but favours midrange detail over high end detail. It is also occasionally a very little bit rough around the edges and needs to be well matched with speakers that don't have any tendency to be bright/harsh. Its mid range focus also means that in the high end, it is not the last word in refinement and in delivering the detail that contributes the sense of air air/atmosphere. So I would say that the vocals will be richly rendered and present, but you will not get every last aspect of the air surrounding the vocalist. The degree to which this is important is very much a matter of taste. In terms of bass, I found the 600a very capable with the right set up.
The 900u is truly possibly the last word in SS refinement, beautifully addressing the high end detail that contributes to air and atmosphere without the slightest hint of harshness. In fact, the 900u is the most velvet smooth amp from top to bottom that I have ever heard, with an addictive tonal neutrality across the entire spectrum. It's perfect in a way. There is no doubt when you hear it that it's a much more expensive amplifier than the 600a. It is not unlike the difference between fine wool and cashmere. The presentation of vocals in the 900u is tonally spot on and surrounded by an angelic amount of air, but slightly less present and rich. I spent a lot of time trying to dial in that midrange presence (as well bass tautness) with the right choice of cables, and got 95% of the way there (I just described these cables in someone's recent post on the M900/c900, btw, if you are curious), but I have to admit there was always something I missed from the m600a, even though its almost impossible to go back after you've heard the m900's refinement.
I never found a synergistic tube preamp that did the trick either (I tried Thoress & Shindo), but maybe that does exist.
So let me put it this way... its going to depend on what your priority is between presence of the vocals and the air/space/atmosphere that surrounds them, between direct engagement and refinement. They are kinda two different was of getting the voice in the room with you. It's far from an "either / or," as they both do all things very well and the 900u does something special almost no other SS piece of gear does, but there is a clear difference in their approach. Where they are similar is in the way that they make sound feel like its pouring out of your speakers and filling your room. Luxman is really good at that.
Sorry to fill the space in this thread with a slightly off topic (and way too long) description, but I actually don't know how to do back channel communication. Happy to learn.
All the best,
J
