[byrne]

My issue us that I don't listen to my records enough, but I need to be able to do it with headphones sometimes. My current setup doesn't allow this, so what is the best way to modify what I have?
 
Turntable: Rega RP3
Preamp: Cambridge 551P
Speakers: Kanto Yumi powered(I know...not exactly hi-fi but I use them for other sources)
 
Since neither the preamp or speakers have a headphone jack what should I add? Is an RCA out > female headphone jack cable from the preamp an option? (Or even a good idea?)
 
Thanks for any help!

[dB Cooper]

This is just a phono preamp connected to a pair of powered speakers?

Last question first: No, I don't think so.

I think you need a headphone amp, even something 'cheap & cheerful' like the Schiit Magni, and then plug the 'phones' into that. A simple A/B RCA switch would allow you to send the preamp output to either the powered speakers or the headphone amp.

Are there other components involved? You mentioned 'other sources'. If you have a full-function preamp or integrated, you could run a headphone amp from tape-out or EPL- out jacks.