Here's a few general rules of thumb:
The further away the CDP is from the power amps, the more likely a preamp will help.
If you have a built-in volume control: if analog you will need the preamp less, if it is digital, you will need the preamp more (digital controls rob the signal of resolution as you turn them down, so you use the volume control in the preamp instead to avoid this problem).
Why CDP and DAC designers use digital volume controls is beyond me. None of them work and a digital designer would be the first to know that...
The further away the CDP is from the power amps, the more likely a preamp will help.
If you have a built-in volume control: if analog you will need the preamp less, if it is digital, you will need the preamp more (digital controls rob the signal of resolution as you turn them down, so you use the volume control in the preamp instead to avoid this problem).
Why CDP and DAC designers use digital volume controls is beyond me. None of them work and a digital designer would be the first to know that...