I've decided to try Tubes

Never heard Primaluna driving Zu's, but I drove my Zu Definition Mkiii's with 5 watt Cary Audio 2a3 mono blocks for a couple of years before trying a $995 Decware Zen and falling in love with it.  I can say with 100% certainty that the current Zu full range drivers thrive on SET power.  The little Zen might be worth a shot, especially considering the incredible support system that comes with Decware gear.  I've owned tons of well respected tube gear over the past 35 years, but to my ears, the 2 watt Decware SE84CKC, at far less cost, is as good or better than all the rest.  

Kinda gives the "first watt" concept new meaning since after the first watt there's just the second watt and that's it.

Linear tube audio has some amps that might work nicely, and they offer a 15 day in-home trial that might be very good for perspective if nothing else.   Best of luck. 

Look for Triode Labs El-84 amps, used, for a stretch of your price. (See 6-Moons review.)  Or, Decware Mini Torri MkII El-84's \the best choice of a top speaker placement specialist I'm aware of (on Def IV's, in Atl.)  (Not me, though I do own Def IV's...Tannoy Churchills, lots of Living Voice, Opera, Totem & DeCappo). They're a good start, you'll might move to 2A3's or OTL's later for a little more extension or transparency but with a loss of bass solidity. - Don't think these smaller output tubes don't do bass.
El-84's were what some of the best amps "of the day/back in the day", Scott Amps, used. Don't count them short. -About 20 push/pull watts. "Good" is  still  Good.
(I too, think the Zu speakers are a bit less efficient than they claim to be.)

One question for Tube owners. How long do new tubes typically take to break in. I've rbeen ad that some high end Gold Lion tubes need a couple of hours of burn in time to open up.  Just curious