Aric Audio? First Watt? PrimaLuna?

jgoldrick, I'm not pretending to be an expert within the First Watt and PrimaLuna arena that you are working within, except to say that they are among the top flight choices. I can however address some of your concerns about going with a tube driven amp -- especially one that comes off Aric Kimball's bench at Aric Audio. I have two of his systems. First the heat issue: it's not an issue. Tubes get hot and the amps get warm, but they are designed to do so and with your cooling fans running and the cabinet doors opened, there is going to be no net effect. He builds his amps auto-bias so there is no longer the need for a tiny screwdriver and a multimeter. Believe me when I say that if I were replacing power tubes every two years at $600/per, I would be in search of another passion. Tubes last a long, long time; some of the best are Vintage or NOS from the 40s through the 60s and while I know it's possible to spend any amount you set out to spend on any thing, I have never approached those numbers, and won't.

But above all else is how they sound. It's why we do this. After looking over your system specifics, I would encourage two courses of action. One, talk to some folks with tube gear who will doubtlessly validate what I'm claiming here, and then secondly but most importantly, get next to one that you can listen to -- with ample time, with your music, in your comfort zone. The sonic difference is staggering. You're in this deep, I believe you owe it to yourself to not leave this decision to the guesswork and opinions of others.  Godspeed.

Why not have Klaus upgrade the Khartago? Depending on budget you could go full Kismet as monoblocks.

Getting in on this rather late... I’d have to agree with Almarg... The Pass XA25 is a terrific amp and well designed/implemented. I have the Pass XA30.8, a little bigger than the XA25 but very similar implementation. I also have a First-Watt SIT-2 and a nice 18-watt 300B SET amp. My speakers are Living-Voice OBXRW’s ~93-db with ~6-ohm impedance and are very tube and SIT-2 (8wpc) friendly, so not a good comparison to your speakers. I’d shy away from the First-Watt unless you have a fairly benign impedance and fairly efficient speakers. The SIT-2 is the most transparent and best sounding solid-state amp I’ve heard on my speakers. At moderate listening levels, it’s terrific and definitely on-par with my 18-watt 300B SET. I also love the sound of the Pass XA30.8 and I believe the XA25 would have the same sonic traits.  While not as minutely resolving as the SIT-2 on my system, it is very satisfying.  And on the 30.8, I've seldom ever seen the meter-needle move even playing at LOUD volumes!  It never seems to run out of power.  All the Pass and First-Watt stuff I've purchased have been through Reno Hi-Fi.  As far as I'm concerned, the best Pass/First-Watt retailer in the U.S.  

Thanks All for the feedback.  I am still waffling.  Since these speakers seem to require a good dose of current, I am wondering if I want to stay on this path or head to the high efficiency path.  Which is a good thing.  If I decide to stay with the B&W's in the long run, then I need to stay with amps that can push them properly.  Otherwise, I need to consider different high efficiency speakers. 
At least, that is where my head is at right now!

I think that’s the way to go. Get as highly efficient speakers as you can that you like and you’ll be able to use whatever amp you want. Took me a while but I’m glad that’s where I’m at now.