If you had this chain: Turntable > Phono preamp > Beolab 6000 you are correct that it will most likely be extremely loud, assuming the phono preamps gain is decently matched to the turntable cartridges gain.

You do not need an amplifier (though some can be used as a preamp). You need a preamp.

Here is a very simple model preamp as an example:

https://www.schiit.com/products/sys

This will allow you to convert the fixed gain output of the phono-pre-amp to variable gain and thus give you volume control.

Thank you very much for the quick reply - very helpful!

I've used one of these before - they are cheap and use no power but simply act as a volume control between your phono amp and the speakers - it's the cheapest option but also great results as its the cleanest path to the speakers. If you're handy with electronics there are remote controlled versions available on ebay but you need a power supply and a case to build them into. Here is  link to the passive one (there are other nicer looking ones around though)