I'd like to provide a little additional information on the Folsom amp. I think there can be a lot of variation in the performance from build to build. For instance, I started testing these chips early on thanks to Scott Clark. I started out listening to one of these cheap assemblies using a Chinese copy of the chip: https://www.ebay.com/itm/Mini-Digital-Amplifier-Stereo-Audio-12V-Amp-Board-15W-15W-Dual-Channel-Speak-Car/183256353617?hash=item2aaaee0b51:g:738AAOSw0lVbDYw~
I think you can find versions for under $4.00 still.
It sounded okay, and I could tell there was a lot of potential. But it wasn't anything super incredible. After I stripped everything off of it and used high quality wire and connectors it was completely different.
These chips are super transparent and sound like whatever you feed them. They really lack their own signature. It's everything else that gives them a specific character or sound.
So back to the Folosm amp. The boards, chips, and implementation are excellent. If building more of them I'd go with his kits over the cheap Chinese amps that you can buy on E-bay that use the same chip number. But again not all builds will be the same. Choice of input coupling caps, wire, connectors, and everything that goes into the build will have an effect. So not all builds will in any way be equal. Like most amps I get much different results with different power cables, Uber Buss in the system, balanced power supplies, etc. Everything matters.
I did feel like the top end on this one that I built out was a little softer than the mono-blocks I made using the amps from E-bay. But I think it was because of one of two things. 1) It could be because I used an input coupling cap with the Folsom amp and with my amps there was no coupling cap used. My pre-amp has 3.3uF Jupiter caps in the output and a .047uF Sonicap (for high pass filtering) on one output. So with the Folsom amp there are two coupling caps in the path. I'd like to try by-passing the coupling cap and hear what happens. Mine aren't stable with the 3.3uF output coupling caps of my pre-amp, but are fine with the smaller .047uF caps. The Folsom amp might be fine on the 3.3uF Coupling caps. I don't know. 2) Or it could be that the Folsom amp will continue to open up on the top end when the Sonicap Platinum caps have burned in longer.
I didn't find it at all forward or in my face. It was very smooth and had a nice sound stage. It is fast though and has very sharp detail though. But if I plugged it straight into the wall with a cheap power cable then that might all change.
With my Super-7 speakers I may actually prefer the sound of my small tube amps with some music as the Super-7's are also super fast and have very sharp detail. There is a lot to be said for system matching. My recommendation is to borrow or try all that you can and decide what works best for your system. And I highly recommend giving the Folsom amp a try. As cheap as it is to build, build one for the fun of it, and keep it around as point of reference. It might even stay in your system.