I've decided to try Tubes

OP,
I would add the Rogue Audio ST-100 to your list. It’s about 3k new but used should be close to your budget. I’ve owned two different Jadis amps and while the Rogue is not quite in that league it has nothing to be ashamed of.

aberyclark,
Like you, I ran various B&K mosfet based amp systems for 25+ years and understand the interest in tubes if you like that warm B&K sound. The best was the ultra high current dual mono amps and last edition preamps before it all went to AV, ugh.  FYI:  ATI amplifiers bought the defunct B&K company as you may be aware. I recently learned that ATI is going to bring back the B&K brand, and I suspect it will be very different and updated with ATI designer influence behind it.  Years ago I eventually moved from B&K to Cary tube pre-amps/amps along with friends wanting to explore tubes. It's been a lot of fun messing with various tube amps, pre-amplifiers, mods/upgrades. Then one day I tried some of the more recent solid state gear and started playing with mixing tube pre-amp front ends with solid state back end amplifiers. It's another combination I waited way too long to try and now that I have, I will always have this combination and continue to search for a great back-end tube amp that can match it in sound quality, dynamics, and full deep bass. I thought I would never leave tube amps, and probably would not have happened until I heard a newer and much different solid state amplifier paired with with a tube preamplifier with really good NOS tubes. It's going to take a hell of a pair of tube monblocks to replace this tube pre and SS amp combo. The only recommendation I can make is to try (borrow/buy/return) different amps with your speakers until you find a match if you loved the B&K stuff. Took me several tube amps and ss amps to find a match that worked well.  Good Luck.   

Older thread, sorry to revive, but yes PL mates very well with Zu.  I currently own two PrimaLuna integrateds and 3 sets of Zu speakers (Cube, Unions, and Soul Supremes).  Check out a few of my recent posts on the Primaluna HP for getting the most out of it.  I would call my Dialogue 2 a great amp for the money, but with KT-150s the HP is simply a great amp period.  My experience is while the Zus are really, really efficient, they shine on all kinds of music with a push-pull at 40 wpc or more, or a nice vintage SS (my Cubes sound amazing with a Marantz 2265) of similar wattage.  I've tried a few 3-10 watt'rs and while they sound amazing at lower volumes and with acoustic/voice (still own a First Watt SIT-2 and 2 Transcendent OTLs), they lose a little punch and immediacy at higher levels or with more aggressive/complex music.

I would recommend the Atma-Sphere M60 Amps, Change the Stock Tubes to better 6SN7 Such as RCA or Ken RAD. You can thank me later :-)

The M60 is a sleeping Giant.