Hello Neil and welcome to the forum!
Did I got it right that you have a Beomaster 5500? In that case the upgrade to Pentas ist just "plug & play" as you can use a "speakerlink" cable to connect them. To put in a rather simple way: When using the connections for active speakers (afaik it is Speaker1 on the BM5500) the signal doesn't pass trough the amplifier of the Beomaster but directly to the speaker where ist is amplified by the amplifier in the active speaker.
The "Beolab family" (B&O active speakers) have now all "Powerlink" connections (DIN-Plugs with up to 8 Pins depending on the speaker) and some older ones (Penta MkII & III, Beolab3000, 5000 and 4500) have both speakerlink and Powerlink connections. There are adapters from speakerlink to Powerlink, but If you change completely to active speakers I would suggest to "upgrade" your Beomaster to a BM6500 or BM7000 as they have Powerlink connections and so it is all plug and play will the whole "Beolab" range.
You don't need to worry about the RIAA pre-amp, it is only used to amplify the signal that comes from the Beogram record player to the Beomaster and has no effect on the signal that goes out on the speaker connection.
I hope this helped an wasn't too much for a first input.
cheers
Foxtrott
"The first thing we do, let's kill all the lawyers."
Henry VI, Part 2 William Shakespeare