Can't comment on why it's better than the 80s stuff outside of build quality, but I did buy one and recently recapped it while cleaning some other stuff up so far (havent done the big caps yet). That said, it absolutely sings now vs. when I first picked it up.
Some thoughts...
I'm not sure the benefit to the DISC mode just yet, comparing it to Phono w/ Tone disabled is very hard to pick out what's shifting, if anything. Has anyone else had experience with it being beneficial? I tested with my headphones and while there was a difference, it was like switching DAC filters, and there was no wrong or consistent answer. DISC had slightly more crispness on the vocals but through the pre-amp I think it sounded more enjoyable. Blind testing and picking moments where I preferred the sound was pretty split and probably depends on the song. Eh...
I don't currently have a subwoofer to attach so using the tone controls and boosting the bass is very useful with my Klipsch RP-600Ms. While I love EQing and tone controls, the drawback is usually them being noisy and also boosting underlying noise. This thing is dead silent, it's almost unbelievable. To compare I also have a Yamaha C-4 pre-amp from the era (much more expensive) and their top of the line EQ from the 80s, and both have faults and add levels of (tolerable) noise. This flaw is easier to pick up with headphones, and I can say that when cranking the bass with tone enabled, I'm just not hearing any noise enter the signal. This vintage engineering really puts something like the Schiit Loki / Lokius to shame, especially since they market those units as being better than vintage despite being noisy as hell.
Speaking of headphones...oh boy. While I have the C4 as a dedicated headphone amp, since it has a dedicated headphone amp built in, the A1 has the headphones built into the tone board and like most integrateds it's an afterthought. Interestingly, though, is just how quiet this thing is
even with headphones. Usually its easy to pick up on junk noise with 300ohm headphones once things get moderately loud - even on my modern headphone amps I notice it with high enough gain, and especially with an EQ in the loop. What the A1 is doing here is nothing short of magical to me. I can crank the bass, and crank the volume to head-exploding levels and it's just eerily silent. The 1978 C4 has a dedicated HP board and has noise, the A1 doesn't - go figure. That being said, the A1 is too big, heavy, and hot to use as a headphone amp, but it's interesting to me nonetheless.
As far as normal music listening goes I'm very impressed so far. My current/previous amp for vinyl is a restored Marantz 2225 and its a night and day difference both in quality and headroom. Even using my Bifrost 2 into the AUX I was able to tell it was my Bifrost 2's sound signature, whereas on the Marantz there was nothing special going on (basically bottlenecking the DAC).
Contemplating what to do about the 18,000uf filter caps right now, and still want to clean & re-thermal the transistors in this. New LEDs should be here this week as well.