So I've had the ZMA for 4 years now and have been running with the original KT66 tubes. I figured it was time for some new tubes and all this talk of the 7581A's got me to order a matched quad of new production Tung Sols.
It may have been that the KT66's were getting a bit tired, and eventually I will get a fresh set of those as well. But I have to say, the 7581A's have glued me to my listening spot in ways I've never experienced with audio ever before.
When the tubes first arrived I warmed them up with Rubber Soul (mono LP) from the boxed set. I like to listen first in mono to see if everything is balanced and how the soundstage presents itself. Everything sounded fine. So I began the "burn my ears in" (and maybe just maybe the tubes) phase. During the first couple of hours I actually began having regrets and that I had wasted my money on tubes that really weren't doing it for me. The sound was both too close and too far away at the same time - muddled is the best way to describe it.
It was about five songs into The Starseeds Parallel Life (trippy mellow ambient stuff) on Tidal (PCM) through my Schiit Bifrost multibit DAC (speakers are ProAc Anniversarys, with a separate SVS SS subwoofer run off the Van Alstine SS preamp) that something unsettling happened. This is the creepiest audio experience I've had recently. I mean, since we're all tube amp listeners here, I know we've all experienced sounds that seem to come from unexpected places, either because it was mixed that way or due to serendipity. Sometimes I swear I hear my wife calling out to me from the other room but it's just some vocal! Anyhow I'm sitting there passively listening while reading some stuff on my laptop where I get the creepy sensation that the leading edge a bubble of sound has just made its way past my ears...
...there is a sensation of eerie pressure and the very real feeling that a comically large, soft/fluffy pair of headphones is being gently lowered onto my head. Now, while I was chatting with mother nature earlier, we chat all the time and I can say that this wasn't about that... ;) ... so I remained glued in my spot for about 10 minutes while the music continued to play lest I lose this bit of magic... but what I discovered was that indeed this effect was persistent, and that I could modulate it by moving my head forward or backwards. I first moved about six inches forward in order to see if I could "swim in the opaque sphere of sound" and on some music this was a wonderful effect and with others it was just too much. Dead-center-mixed vocals and other midrange-prominent instruments can sometimes feel like they are coming from within my head, which is a nice parlor trick but not the most relaxing experience. On music where this effect was too much, moving backwards ameliorated it nicely, making the prominent midrange sound "out there" more.
Steve - have you done any serious listening to the stock ZMA with 7581A tubes? I'd like to hear your perspective on these since I very much trust your ears.
Tube amps have ruined my scientific certitude when it comes to some things audio (I am a working scientist). Psychoacoustics is a fascinating subject to me. As you can see above I have a dedicated listening room and I have put up room treatments. The room is large and opens to a larger room, effectively opening to an infinitely large space. So the bass is wonderful in this room, no more annoying nodes. But the imaging is insane, partly because tubes/Decware, partly because ProAc, and partly because of symmetry. I do ponder the fact that 99% of studio produced music is a complete utter fabrication of whatever the dude at the mixing desk decided to do when putting things together. Considering that some more maybe a lot of mixing consoles are running noisy and/or out of spec components and for a thousand other reasons, the idea that what we audio nuts hear on our awesome systems when we are in the sweet spot is what is actually intended is ludicrous to me. Once I came to the realization that hi-fi isn't about a straight wire with gain but more about a pleasing aural experience, I freed myself from worrying too much about measurements, or at least taking them with a large grain of salt. I believe what makes Decware amps (and other high quality tube amps) so great is largely due to simple circuits with short signal paths. Something gets bleached away with solid state amps, at least the in my experience, which is admittedly limited. Because I've found audio nirvana with the ZMA I'll likely stay here, maybe indefinitely.
In closing this somewhat rambling post I'll come back to tube rolling. I also have some nice NOS 6CA7 tubes that I will give a more serious listen than the first time when I gave up after a few hours, not liking the less lively sound than I heard through the KT66s. One of the reasons I got the ZMA was because it would run with different tube types. This is my first experience with a 6L6 type tube, it was EL34/6CA7/KT77 on a Dynaco ST70 before I got the ZMA, and KT66 since I got the ZMA. Putting in a different tube type makes it sound like a different amplifier - not necessarily better or worse, but different. I now "get" voicing... having sort of scoffed at it back in my more reflexively objective days.
I've also come to realize that some people just plain have better ears than I do, or at least know how to use them better than I. I do not trust my ears, as they say, in the least... but I do trust the ears of certain other people whose products make great music... all of my gear has been bought sight unseen and so far I have not regretted anything.
In summary, the stock ZMA rules, the 7581A tubes produce liquid honesty with just a small asterisk by honesty that says "beware that the 3D holographic soundstage may not have been what was intended on original recording."
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