First Watt and Aleph amps with Tortuga passive preamp
Several threads, here on Audiogon and elsewhere, that discuss these amps recommend high-gain preamps and/or sensitive speakers. My son and I have 3 Nelson Pass amps that we share back and forth - an F5 clone (25wpc, 15dB gain), an F5T (50wpc, about 20 dB gain), and a pair of Aleph 2 mono clones (100 watts each, 20 dB gain using unbalanced inputs). The Tortuga preamp has no gain, of course, and my ML Source electrostatic speakers have 90 dB efficiency.
Even the F5 amp has plenty of gain and plenty of output power to make the Martin Logan electrostats sing, and I have never wanted more volume. The other two setups, both with higher gain and higher output power, are a little better at controlling the bass perhaps and they will blast the neighbours with a higher volume level if I so desire.
Nelson Pass states that, if 25 watts isn't enough to power your speakers to a satisfying level, then maybe you should get new speakers. My experience seems to agree with this, and I will add that a passive (zero gain) preamp is definitely up to the job, without any hint of the music having the life sucked out of it, as some people suggest. The Tortuga has no buffers, so careful consideration of source components and amp(s) is a must, although Morten of Tortuga says that the majority of gear is OK. In my case, that Aleph 2 amps (10K input impedance) were possibly incompatible but I took a chance and bought them anyway. Turns out that there is no problem with this match-up. At this point, I tend to prefer the single-ended Alephs in my system, but that could change - the F5 and F5T (both push-pull) are terrific amps, too.