1.). The MJR-6 is included in simulation tests here and is also mentioned on some of the links. It is in French, and I only understand a little, so some of my criticisms here may be unfair. There is a xls file giving simulation results for more than 20 designs, with points awarded for the various results. The MJR-6 appears to do well apart from having 4 points deducted for the output stage mid-point voltage temperature drift, resulting from variations in Vbe and current gain of the input transistor, mostly caused by slow changes in air temperature, any self-heating related to the input signal being extremely small. For a capacitor-coupled design this has no effect on output offset which measured somewhere under 0.1mV, and depends on the leakage current of the capacitors. The drift only affects the maximum peak output level, and should have no effect on sound quality below clipping. Unless a stabilised supply is used the supply voltage is not constant, so the question of whether the output stage voltage is at the mid-point is meaningless, and it could even be argued that if the temperature is high this must be because the power output is high, and so the average supply voltage will be reduced, and a reduced 'mid-point' voltage at high temperatures could therefore be an advantage. (If I ever make a commercial version I must remember to mention this excellent design feature!)
Another point is reduced because supply rejection is only 90dB, which I guess is calculated at 100Hz. This could easily be improved if required, but the supply noise may already be 40dB below maximum output, so then the actual supply effect at the output is at about -130dB. The threshold of hearing is at sound level 20dB at 100Hz, so assuming 100dB maximum sound level, the supply effect is probably 50dB below the threshold of hearing. The highly regarded JLH class-A amplifier was found to have only 62dB supply rejection, but even this is more than adequate at 100Hz.
On the plus side the distortion figures are among the best, with only two others having lower third harmonic, and both of those have far higher quiescent current, operating in class-A at the signal level simulated. From the present top 16 (Jan 2007) I believe only the MJR-6 and one other design are not simulated in class-A.
Even so, one more point is deducted, apparently because the distortion harmonics are not related in the way sometimes claimed to be necessary, for example in the infamous Cheever paper, but worrying about what sort of distortion sounds best becomes entirely pointless once it has been reduced far below audibility. (The MJR-6 distortion, both simulated and measured, is primarily second harmonic.)The MJR-6 was never intended to be the best possible design, I just wanted to make a simple and reliable amplifier suitable for less experienced constructors, so maybe I should be happy with this rating, but the marking system chosen does lead to some strange results, and some of the designs given higher marks appear to have real problems. There are a few good designs, including one of my old favourites, the JLH class-A (which scores only 12), which achieves adequately good results without unnecessary complexity. I once made one myself, and was perfectly happy with it for many years.
A factor common to many of the designs is their use of symmetry, which often appears to be pointless and unnecessary. I have a theory that these sort of designs are partly a result of the widespread use of simulators to design amplifiers. Excellent results may be possible with some highly symmetric designs by assuming exact component matching, which is easy in a Spice simulation but not so easy to achieve in reality.
