I’ve created a small Excel tool to help you calculate the actual frequency, provided you are able to measure the real frequency deviation. I reduced the read frequency by -2 ppm by coating the oscillators on the motherboard with DUE-CI V-66 Isolating Lacquer Spray.
Siglent only allows adjustment of the internal timebase offset on its top-end models (e.g., the 6000 or 8000 HD series), a feature that is very simply implemented via the PLL. In any case, the cost savings on the SDS800X are not significant, since the simple oscillator used costs €1, whereas a 0.3 ppm VCTCXO costs €8.
Likewise, on the lower-end models they have removed the physical fan-speed control, even though it remains available in the service menu.