You are right. An 8-bit DSO cannot measure distortions below some -48dB because of the limited linearity of the ADC.

Likewise, even though modern waveform generators have up to 16 bits, their fast DACs cannot be ultra-linear and this is most likely also true for the analog amplifiers inside the generator.

I don’t have a SDG1000X or 2000X, but I can show you what to expect from a 16bit system with my SDG6052X. It should perform pretty much the same as a SDG2000X at low frequencies.

First a 20kHz reference measurement with a 16bit DSO, which is optimized for low distortion and noise and can cleanly measure down to -96dB:

SDG6052X_THD_20kHz_Pico4262

An ideal 16bit generator would have a THD of no more than -96dB.

From the screenshot above we can see that the strongest harmonic is the 2nd and it is only ~82dB down.

THD up to 1MHz is measured at -78dBc, hence some 16dB higher than the theoretical ideal value.

Total Harmonic Distortion measured at 1MHz bandwidth is still low at 0.012%.

Now let’s try a similar measurement with the Siglent SDS1104X-E:

SDG6052X_THD_20kHz_SDS1104X-E

The Siglent doesn’t support automatic THD measurements, but we can still compare the strongest harmonic. For this we need to calculate the difference between the fundamental at 20kHz and the 2nd harmonic at 40kHz: +8.95dBV - -37.9dBV = 46.85dBV; if this were the only harmonic then THD would be measured as slightly less than 0.5%, hence much higher than it actually is.

Compare this to the 82dB we got with the 16bit DSO…

So it makes no difference for distortion measurements on an 8bit scope whether you get the SDG1000X or SDG2000X. For Bode Plots, a SDG2000X can go up to 120MHz, whereas the SDG1000X is limited to 60MHz max. Since you seem to be mostly interested in audio, this wouldn’t matter either.