[G0HZU_JMR]

For your needs you could also consider the Analog Discovery 2 as it contains a 14 bit ADC (100MSa/s) and can accept inputs up to something like +/- 25V. It's not going to be good enough to satisfy the needs of serious HiFi enthusiasts but you get a 10MHz spectrum analyser that works quite well down to audio frequencies. Sadly, the prices of these have gone up a lot recently, they used to be about £200 new but now cost about £400. It might be possible to buy a used one at a sensible price. To prevent damage, some care would be obviously be needed if you coupled it up to the internals of high voltage valve stuff but that probably applies to alternative analyser options as well.

The main disadvantages of the older HP gear is that it is big and heavy and quite restrictive in what it can measure. With a modern DSP solution like the AD2 you can look at individual harmonics or spurious terms. You can also use it as a scope or a crude waveform generator.

A few years ago there was a glut of Keithley 2015 THD bench DMMs on the market and these could be purchased for under £300 each. This meter has built in DSP and can do THD or measure individual harmonics. I think the residual harmonic level is -87dB which ought to be good enough for all but the most serious applications. The distortion performance of the internal signal source isn't very good on this meter though. Prices of these meters seem to have rocketed in the last couple of years and I guess this is down to supply and demand.

A basic rule of thumb is that the internal harmonic distortion level of the test receiver has to be about 20dB better (lower) that the distortion in the device under test if you want to measure an individual harmonic to within about +/-1dB uncertainty. The signal source obviously has to be very clean too.