The mk6 is way advanced from the sba in my estimation.
I see a lot of points being made about different aspects of each horn's features. But nobody has mentioned what I think is a major factor in what it takes to really get around, technically speaking, on each one of these horns.
The thumb rest, and the thumb hook. Both of these features were greatly improved upon on the mk6. Now, you may not think that these two features are very important, but I've found these to be two of the most important parts of the horn. So let me attempt to explain the importance of these two "keys", as I use them.
Starting with the thumb rest. This may be the single most important feature on the horn. The fingers of your left hand have a lot to do when playing. How many keys does your left hand control? Fourteen? Fifteen? The little finger spatula, low Bb, low B natural, low C#, and the G# key. The keys on the left hand stack, C, B, Bb with the bis, A and G. The front high F, the thumb octave key. Now you've got your palm keys, D, D#, and F. And if you're playing a horn with the low A key, count that one also. So how much of the horn's weight are you supporting with your left thumb as you are playing? Not too much, but it depends on the combination of notes that you're playing, and the tempo of the tune. The thumb rest is of supreme importance if I'm playing a very difficult passage at a fast tempo which only involves the notes of the left hand, something which, lets say, starts with the stack notes and rapidly jumps to an altissimo G or a high F or one of the other palm keys. And if I want to quickly repeat the phrase a half step down, or up, I'll still be using the left hand. At this point I'm holding the straight horn out in front of me, supported by the leverage of the thumb HOOK against the neck strap. My left thumb is firmly planted into the thumb rest, enabling me to use my left hand to leverage my fingers and palm as seek to find the exact keys which represent the tones that my ear is hearing. The thumb rest frees up the fingers and the palm, a very important job in the negotiation of difficult passages.
The thumb hook has a slightly more blue collar, but still a very important, job. I use it to support more of the horn's weight, at times I'll support all of the weight of the horn with the hook for brief periods. We're also dealing with the right hand stack notes, the D# and low C plates, the side F#, and the side C, Bb, and high E. Now, I mention all these notes for a reason. Let's take these notes, and combine them with the left hand notes and use them to improvise a chorus of rhythm changes. And let's take the tempo to the ceiling, 400 beats a minute. You'll be doing a fantastic rhythmic tap dance with both hands and both sets of fingers. Now what part of the horn is holding down the fort, keeping it still while all this is taking place? The thumb hook, and the thumb rest, both working in conjunction with the neckstrap. The problem is that when you're holding the horn solely with the thumbs and the strap, while fingering intensely, the horn tends to want to flop from side to side. This will hinder execution. So this is where I see the definate advantage of the mk6. The large round thumb rest and the big square hook help to keep the horn centered when negotiating diffucult passages at fast tempos. Sure, you can play fast on the sba or the ba, but I feel that the horn is more stable in my hands with the mk6 setup. And this thumb rest and hook setup is copied on pretty much every modern horn today.
