I should state that I jumped from a very mid-fi setup to my current system all in one shot, so I can't really give a meaningful subjective review. I also haven't heard many systems at this level, so I don't have much to go off of for benchmarks. However, I'll try to give my perspective.
Note that in the photos the equipment stands are extremely temporary, I'm playing with layout before building a "final" stand design. You can also tell from the lack of baseboard what my work priorities are!
Both the preamp and M-125s were a real pleasure to build. You can tell the level of detail that went into each and every aspects of the kits. For working on the M-125s I made up a little stand out of plywood, some pictures of it are below. I HIGHLY recommend doing this when building these amps. The thought of anything touching the day old paint on the transformers was not pleasant to me, and I wanted a nice stable platform to work off of. There are probably lots of other ways to do it, but the design shown was very stable and would also fit with the tubes installed. Well worth the 15 minutes it took to make.
So, the SP-12/PH-12 and M-125s are driving a just completed pair of Zaph Audio ZRT 2.5 speakers in 65 liter ported enclosures. These speakers use the Scanspeak Revelator series of drivers. The cabinets, which are truly works of art, were built by Lee Taylor. I was going to build them myself until I discovered that Lee offered them, and it turns out he's only about 1.5 hours from me. I'm a decent carpenter, but these cabinets are the result of 50+ years of his cabinet building experience and available tooling; I would have never come close to this sort of quality.
On the analog side, I have a Sony PS-X70 TT with a Denon DL-103R cartridge run through a K&K step up transformer before heading into the PH-12. The last piece I really need to complete this system is a good CD player. I'm currently using an old Yamaha changer, and it's finally one of the last weak links in the system. Would love opinions on good options for CD players these days if anyone has any!
So, in all honesty, the Zaph ZRTs are such a radical change from what I had before (Paradigm Monitor 3s), that it's hard to know exactly what the amps are and aren't doing. I *can* tell you that the system sounds absolutely spectacular!! The level of detail that comes through is just incredible, and it's completely non-fatiguing. I've never heard a system with this level of clarity that didn't sound somewhat harsh and metallic at the same time. There have been a handful of tracks where I have been able to clearly discern lyrics that have never been comprehensible before; I think that speaks volumes by itself! Low end extension seems impossibly low and clear for 7" drivers. The room these are in is about 36' x 28' with some halls and stairwells, so it's not your normal listening environment. I didn't want to have to use a subwoofer and its accompanying challenges in my system, and it turns out that I really don't need one. The other interesting thing is that the Zaphs are VERY efficient relative to what I was used to. I'm running 4-5 steps down on the main attenuator to reach about the same level as before. In hindsight, 125 watts is likely overkill, but better to error that way I figure. I'm still letting everything break in, but I have little doubt that the M-125s could reach extremely high SPL with these speakers.
I had some friends over yesterday, and everyone, including some folks that have never really heard even decent gear, concluded that vinyl sounded better given my current CD player. To me, I think that just means my CD player is in need of improvement; I think both formats should sound spectacular. However, I think it also speaks volumes to the quality of the PH-12, and in all honesty, the 103R cartridge too.
In any case, some pictures are below. I look forward to learning more from all the great individuals at this forum!