[snoogly]

I plan to use my PLC1 with a pair of active loudspeakers, though due to constraints imposed by the layout of my room, the cable length between the preamp and speakers will be 20ft.

I realise that I might be pushing the limits here, but do you think the Blue Jeans Cable LC-1 low capacitance cable would be appropriate? Capacitance is rated at 12.2 pF/ft.

https://www.bluejeanscable.com/store/audio/index.htm

https://www.bluejeanscable.com/store/audio/LC1-design-notes.htm

Loudspeaker specs: http://www.avihifi.co.uk/pdfs/DM5_User_Instructions.pdf

[CIAudio]

Hard to say as the specifications for the speaker don't state the input impedance.
Just give it a try...

[RDavidson]

Is your source output voltage at least 2V? That'll help too. Yeah, just give it a try.

[snoogly]

Cables came, works a treat

Now I'm just waiting for my VDA2 to be repaired, so I can enjoy music in its full glory.

Cheers!

[ACHiPo]

Cables came, works a treat

Now I'm just waiting for my VDA2 to be repaired, so I can enjoy music in its full glory.

Cheers!

Now that's a selfy!

[snoogly]

It looks better than it tasted ... Blindly poured from a bottle of tequila instead of rum, then accidentally added 'orange' coke instead of normal coke. Truly one of the worst things I have ever drunk, but I'm sure it could be a hit drink somewhere ...

Festive greetings to one and all 

[Speedskater]

For long RCA interconnect cables, the Blue Jeans Cable LC-1 interconnect would be my first choice.
But then with a passive pre-amp and speakers with an unknown input impedance, it's hard to say.