Quad 606 - highly regarded vintage amp (current dumping) 1983 - 1993, replaced by the almost identical 707 & 909I don't buy products anymore with the assumption that they perform better than advertised.
THD = 0.03% = SINAD -70.46db
Signal/Noise = 105db
It's a great sounding amp - current version of the desing is the Quad Artera Stereo
THD = 0.03% 20-20kHz @ 100W (0.003% 1kHz 100W) = SINAD -70.46db
Signal/Noise = 115db (140W A weighted)
I would not call it a state of the art design - but I would call it an excellent audiophile / high fidelity amplifier
SINAD is not everything - the devil is in the detail.
A SINAD of over 60db is OK, as long as the signal/noise is over 100db - if these two criteria are met, then there are heaps of other parameters of amplifier performance that should be considered more important.