A friend of mine brought over his homebuilt battery powered gainclone and his battery powered SI 5066. Here's his description of our listening session:
I took the Sonic Impact board out of its housing and popped it in a Hammond cast-aluminum chassis, added some nice RCAs, etc and powered it from a 12V battery. This was broken in for over fifty hours. We compared it with
A DIY "Gaincard"-like LM1875 inverted chip amp (20 watts)
The Flying Mole digital amps (160 watts)
A Sun Audio 10Y SET (1.5 watts)
The speakers were Altec 755Cs in 50L sealed boxes, and the upstream digital was a fancy transport linked to a DIY TDA1541 DAC, plus a transformer volume control by Stevens and Billington.
Alas, the Sonic Impact was by far the worst of the bunch, with an odd tonality that reminded us of CD players circa 1990. Yes, it had good extension and imaging, soundstage and detail, but dismal tonality.
We swapped in the LM1875 amps, which are also low-price winners since they have about $40 of parts in them, battery powersupply not included. This was so much better that there was no more A/B testing with the SI!
We also used the Flying Mole amps, tastefully modded by swapping an input cap and bypassing the volume pot. Better than the SI, and way more power, but it sounded "hi-fi" and were not as emotionally involving as the LM1875 "gainclone" amps.
The best were the 10Y SETs, which had their own brand of delicious presentation. Maybe it wasn't "accurate" but it sure was tasty! Alas, with only 1.5 watts, driving the 90db speakers was a bit too much. They went back to their old homes, driving a pair of TAD compression drivers in a biamped system.
Conclusions: The SI is cute but not worth spending time with. The Flying Mole amps are decent, and a good value (about $500 for a pair of monoblocks), especially as a source of big power. The LM1875 was worthy and emotionally engaging, and very simple and cheap, the best of the solid-state efforts.
This we could have a long-term relationship with, and did, listening for several hours into the night. Old-fashioned triodes still rule as Kings of Tonality, and we'll all just have to keep living with their expense and low power for a while longer.
Anyway, that was our experiences.