CD standard output via RCA is 2 Volts and the HDCD standard output via RCA is 2.2 Volts, many don’t play be the rules. Some are over and some are under.
Yes that was the Redbook standard way back in early 80’s when it was first stated by Philips.
For the last 10-20 years I’ve found "the majority" are way over, more like 2.5v even to up to 6v on some dacs I've seen is the output these days.
Only few tube output and "boutique" ones I’ve seen are under 2v
And as far as the amps input sensitivity the majority only need 0.5 to 1.5v input for to be driven to full output
Cheers George