I think Lon will give you a better answer than I would, but I can share my experience and thoughts.
I'm a bit of purest. Back in the day, before the ZTPRE was finished, Steve kept telling me I needed a Preamp to get that extra density (that was the theme then, especially after Reel to Reel masters showed us what density really was). I like density, it brings you close to the music, but I kept feeling like anything we added to the chain took me a hair away from the purity of DAC to Amp. Steve sat me down with the Magic Box (now called the ZBIT) and his best Preamp at the time. I felt that both added something, but also took me a hair away from purity.
I dislike this phrase "it's like a veil was lifted". It seems overused, but in my example, it's the reverse - it's like a veil was added between me and the speakers. But I'm talking like a microscopic, made from spider silk veil and only someone as nutty and tuned into a system would notice (looks at self in mirror - yep). The trade off was more density (though I didn't hear it that much that night), and being able to ride the gain to get your system to "wake up" which is sounds like you need.
Since then Steve has made great leaps and bounds with the 25th amps and mods, and the ZTPRE has been released and I've honestly not revisited this test to see if the
current amps with the
current pre takes me that microscopic step back from "purity". I believe some others have said they couldn't be without the new Pre, and here I am still running my (admittedly) low voltage Direct Stream or NT-503 directly to my 25th modded Zen and 25th Modded ZMA and I'm still blown away by how close to the music and engaging this is.
Also note, my ZMA has had the XLR inputs added, which is basically a ZBIT built in, and I do use it sometimes because the Decware amps just love having that extra voltage on the front end. And I don't feel like I need the gain for my speakers, so my needs are different.
I'd really like to bring a ZTPRE home and revisit this purity vs density with current amps and current pre in my home setup and see how this changes my mind. Pretty much everyone says it will - but I'm stubborn and need to see it myself. Especially since I can't afford a ZTPRE right now.
Anyone in the Chicagoland area with a ZTPRE wanna come over for late night listening session and split some hairs...or spider silk with me??