Quad 2905

I just purchased a used pair of Quad 2905 speakers. Would they benefit if they were plugged into a power conditioner or regenerator?
Good question. I wouldn't worry about it. Money may be better spent on new amp or preamp or DAC. 
Hi, soundsrealaudio is correct, but if you have components you’re happy with a Power conditioner isn’t a bad idea. Especially if your utilitiy isn’t very good like here in NY.  I use a Richard Gray, which I’ve seen here on audiogon offered for a decent price, with Pangea 9 power cables on my 989’s.  Many will say that’s overkill, and it probably is, but I did notice an nice improvement in the base.  And, just so you don’t think it’s none sense, my wife heard it too and she’s not easily duped.  I went with the Pangea’s ‘cause they were the best bang for the buck, and the retailer had a fair return policy. 

Enjoy - few audio experiences like the big QUAD’s . 
Congratulations with some of the best speakers money can buy (I love my 2805s). The power is used to charge the membrane, amd once that is done it only needs to top that up. You can hear it when you pull the plug (deliberately or accidentally): the speaker will continue to play for quite a while until finally bass and level are increasingly reduced. So I don’t think anything fancy is needed - just surge protection.
I don’t know about the current US importer, but you can always ask technical questions at the Quad UK website and you will usually get an answer from their Rob Flain himself.