How Much Power Do Vandersteen 5a's Really Need?

Thanks for all of the input. And thanks very much for the offer of help Tomic, I may reach out for setup assistance if needed. I do understand that the manual has a lot of setup information per a discussion I had with Richard Vandersteen, about these, yesterday. Hopefully that is of significant assistance. 

The room they will be in is very large, The listening area is about twenty feet wide by 15 deep. However, this is only 40% of the "great room" that the listening space is located in. Behind it, is the fireplace area, the dining room, and the kitchen. The ceiling are also vaulted.

I'm hoping to power these with my wonderful Conrad Johnson ARTse tube amp, with 140wpc per side, and a ton of gain. 

Listening volume will range from relatively quiet acoustic and singer songwriter selections, to now and then, loud, live rock concerts at something like at least semi-realistic volume. 

people generally seem to be throwing a LOT of power at them. Am I missing something? 

Yes: the logarithmic nature of power. 10 dB doesn't sound like much but its TEN TIMES the power. 

Look at the spec- they are 87dB. You can let these guys blow smoke up your a$$ or you can save a ton of time trouble and $$ and cross them off your list.

Never buy speakers under 92dB. Just don't.

Although I am firmly entrenched in the "more efficiency, the better" camp, 140 tube watts should be fine to drive these speakers.

Oz

I owned 5A’s for many years and drove them with both tubes and solid-state. 100 W should be fine for normal listening, but if you want to party they can really suck up the juice especially in your big room

With your new speakers and your existing amp you have a road racing Porsche. Hyper low distortion , in room real world bandwidth and superb stereo imaging. IF you want loud hyper efficient get a dump truck - it’s a different task at rock concert levels in a large space.

my offer of help stands, Richard invented your great speakers - no better source, glad you spoke with him. He answers the phone, since 1977