BL5s take the analogue input, convert it to digital (via ADC), apply it's sound correction DSP stuff (DSP), then convert it back to analogue (DAC) to make the sound you hear.
Analogue -> ADC -> DSP -> DAC -> Amp/Cones
The ADC (Analogue Digital Conversion) used in the speakers can handle a 2VRMS signal only. Any more than this can lead to clipping/distortion. Having said that, some people do use 2.5VRMS etc. otuputting preamps without problem.
The line-level audio spec was supposed to be 2V RMS in all devices but in the world of consumer audio, grabbing attention is better so the signals are higher. Even CD-players, Blu-Ray etc. are often off the charts (I've seen 5.5V RMS on spec in a well regarded audiophile brand!).
The analogue audio output level should be in the spec sheet for your preamp or phono section.
If it's 2V RMS or less you should be right to connect it up.
Be careful with tube gear especially as 20V RMS max-out on a phono section isn't unusual (but then again some are at or under 2V RMS).
What phono preamp are you using?