This quote was so informative & succinct, I had to repeat it here:
For what it’s worth, the Rose has 3 factors that have different effects on the delay time and/or sample rate:
1) Delay time setting (via the Delay knob or Tap Tempo). This doesn’t change the sample rate, only the number of samples in the delay line, so it doesn’t make any difference the “fidelity” of the sound. At the nominal sample rate of 100 kHz (with modulation and delay multiply off) you can get up to about 10 seconds of delay time, with no degradation in audio quality.
2) Delay multiply: This cuts the (actual) sample rate in half (or more, depending on the Delay multiply setting) while leaving the delay line the same length. In effect, this also doubles the length of the delay time. At its more extreme settings, this will start to degrade the sound a bit (we stop before it gets too bad 
).
3) Modulation: This continuously varies the sample rate, also changing the delay time. With delay multiply off, this goes from about half the nominal rate to double it, doing the opposite to the delay time.
