How accurate is the Audiolab with respect to flatness of frequency response?
For fixed level FR plots, it's really accurate. 9 (manual software controlled hardware switched) ranges from 100mV to 100V RMS with 8 bit sampling (256 levels per range) and a true RMS converter, 492 sample points in a 20-20K log sweep. Digitally generated swept sine, variable level. Linear and log sweeps with step and start/stop freqs. It doesn't care about the actual frequency of the sweep signal- it measures the amplitude of it. Because the only signal 'should' be the one it generates and it knows what that is and is synced to it.
To give you an idea, these are a few plots of one of my Denon preamplifiers (PRA-1000) and a Sony preamplifer (TAE-77esd) channels offset for clarity at around 1V output. FR deviation of those preamps is virtually one LSB's worth (1 pixel=10mV on the 2.5V plot). These plots are 8 years apart and the calibration routine had just been run IIRC.
The loop-back is hardly flatter than that Denon!
There's a 10-20KHz loopback on the 1V scale, DC coupled. The cross hair shows a few pixels above 0.5V, so a 4mV approx deviation which is essentially a run to run variation.
Certainly no AP, but I've had the old girl since 1996 and what it does actually do, it does well.
AudioLab is tethered (via serial RS-232) to an old W95 box that sits on my network in the cupboard below my bench, so I can pull the data files off it. I've recently (thanks to Blumlein88) been able to finally get the raw data values into excel to create damping factor vs frequency plots externally (doing an unloaded vs loaded sweep). It can also measure capacitance, inductance and do impedance sweeps.
For distortion I use a Leader LDM-171 (quick tests to -80dB max THD/S/N), my Panasonic VP-7723a or PC sound card front ends for low level FFT. Other gear on my bench are a few CROs, A DSO, a Kenwood FL-140 W&F meter for TTs and Tape Decks, Freq counters, PSUs, audio oscillators, a Panasonic VP-8177A Sig Gen, DMMs and some hot, pointy ended things for melting holes in plastic.