Low output gain amplifier for ZU 101db Efficient speakers

Honestly, with that much gain in the pre-amp, all you really need is an impedance converting amp. You don't need an amp that provides ANY voltage gain given the extreme sensitivity of those speakers. You're close to getting away with running the output of the pre-amp straight into transformers and right into the speakers. The amp I'd be looking for would be one, maybe two, stages at the very most. It would have a mediocre damping factor of 15 or 20. 20 watts through those speakers would literally deafen you in very short order. It's not hard at all to get VERY respectable distortion out of a single ended design with the kind of power you need. Speakers like those are EXACTLY the kind of speakers Nelson Pass had in mind when he designed the F4. The problem with the F3 is it has a low input impedance that might not mate well with a tube line stage and it's got a weak damping factor. But, it does have some modest gain you might want. The problem with all the old First Watt amps is Nelson only builds 100 of them. They're like hen's teeth to find and they tend to find homes they never leave. The upside is you can build you an F3 or F4 for about $1000, and if you don't like either, it's a very small investment to just yank the amp boards and mount new ones; like maybe $200-250. That's a hell of a deal for a totally different amp. The J2 is still for sale and that's well known as being an excellent amp for Zu speakers. In any event, it would be worth looking over the specs for those amps at the First Watt page and maybe reading through the owner's manuals. Those are the kinds of specs you're going to be wanting to look for. Modest power, mediocre damping factor, decently high input impedance, possibly single ended, low noise. I wouldn't sweat crazy low distortion numbers because you're not going to be needing so much power that the distortion will rise much. 

I have read a couple of faverable reviews with the J2 paired with ZU speakers. RENO hi fi has a couple demo j2s for sale

He qualified it by saying that his amps sound better with an active preamp. He basically said that it’s an unexplained phenomenon.

Of course he did, that’s one of his bread and butter items, he’s not going to shoot himself and his company in the foot, left himself an "out" by saying "an unexplained phenomenon" BTW Where is this quote?

Cheers George

ALL of Nelson's pre-amps are active in that they use transistors to buffer the output at the very least. That doesn't mean they have gain though. Nelson likes to use active buffers downstream of the volume control in order to avoid response variations going into the amp produced by a volume control who's impedance is directly seen by the power amp. Some of his amps have very low input impedances. 

Some of his amps have very low input impedances.

Yes granted, this is the reason why an active pre is needed, if they were >50kohm then a low impedance active pre is not needed, remember the Industry Standard for input impedance was 47kohm or higher, but sometimes if the input stage is bi-polar this not achievable, but it is with fet input, then it’s up to the designer to make it so.

Classic example of a high end poweramp here in Australia having only 1kohm input impedance, only their own preamp with 10ohm output impedance would drive it, all others especially tube sounded like crap, so no need to say they also sold many premaps with that poweramp.
But here the crux, inside that poweramp was a secrete link that when pulled made it 68kohm input impedance, which I exposed online, then owners were free to use all different preamps with it including passive. It’s called shady marketing, good if you get away with it as it makes sales for their preamps.

Cheers George