Sorry for the delay getting this review in. I appreciate the AMR/iFi folks sending out this piece of gear for evaluation. Very generous!
I've been most interested reading about other members systems, and in particular seeing photos. The photos help give peoples comments about the iTube and how it works in their system perspective.
Although my system does do some things well, it does not do probably the most important thing a quality stereo should do, and that's recreate a believable 3 dimensional sound field of a live un-amplified acoustic performance. One that is completely transparent so the listener has no idea where the music is coming from, and instruments can be placed both behind and in front of the speakers themselves, as well as beyond their width. Rather, it's quite 2 dimensional. There is front to back depth, and width, but because of the speaker design, no 3rd dimension.
My speakers are control room monitors (UREI 813C) and were intended to be mounted in an infinite baffle. Because that wasn't possible in my home, I needed to build a pair of subwoofers to reinforce low frequencies.
The trade off for not getting 3 dimensional sound is I do get big, effortless, dynamic, coherent sound. Always a trade off in this hobby. At the listening position, it's measured remarkably flat from 25hz-13khz. Bass is solid with no hint of tubbiness, midrange is as natural as I've heard, and highs are smooth and clear, though rolling off above 13khz. I never considered horn speakers till I heard these.
I won't go into all the details, but I was growing concerned that bi-amping (the subs are passive) with 2 solid state amplifiers that have modest input impedance (33k ohms each), and an active tube preamp with normalish high output impedance (3-4k ohms) might not be the best combination. I didn't need the gain of the active pre (speakers are 102db) and I am not a fan of transformer coupled tube amps. I was able to borrow a well designed solid state preamp, which got me a nice low output impedance, and sure enough dynamics and drive improved immediately, but the sound was too hard for critical listening. I then borrowed a Berning ZOTL preamp which has similarly low output impedance (<2ohms) and is 12au7 based. Okay, now I'm getting the best of both worlds, except for the cost. A little interweb research and talking with a couple friends in a local diy club turned up the idea of a buffer, and within short order I had an iTube on the way.
I initially installed the iTube last year between my PS Audio Directstream DAC and the amplifiers without a preamp, as the DAC has volume control. Results were good. Not great, but good. Then, I borrowed a First Sound passive preamp and installed it between the DAC and the buffer. Magic, much to my surprise, that adding a passive and another set of interconnects could make such an improvement. The system now had the drive and dynamics (low output impedance), plus the natural richness of a quality tube preamp (iTube), and whatever magic happens in a passive preamp (that I don't understand).
So, when this tour came up I thought I'd sign up and see if there will be any difference in my system with the iTube2 since the original was so successful for me. I did several days of listening evaluation, and then took it to a friend's home for measuring. I only examined it in the 0 gain buffer mode. I cycled through the preamp and 9db buffer modes with the original, but quickly preferred the 0 gain buffer.
I've always used the original iTube without any 3D filters and the Digital Antidote turned off. In my system, that has the most natural presentation. The 3D filter in particular, just blurs instruments (piano in particular) in the image. So, I decided to start with the 3D in the "disengaged" position, Bass boost "off", and output in "Classic". After many cycles through the same tracks at the same levels, both alone and with my wife helping do blind comparisons, we both came back to this same starting point. We felt is was the cleanest, most natural of the combinations. This sounded very similar to the original, with just a little less gain in the high frequencies.
The SET and Push/Pull output settings each sounded more restricted in the high frequencies than Classic, with SET being the most restricted. Bass did not appear to change at all with any of the 3 outputs. Fortunately for me, these observations were apparent when we measured it. Because of what we were perceiving as restricted highs, the iTube 2 did open up more of the music library for casual listening. Many of the poorly made cds of our youth were far more enjoyable. Still not good, but more enjoyable. We regularly switched from Classic to Push/Pull when a bad (but beloved) cd was played. The Xbass wasn't necessary with my system, but I could see this being a good filter for smaller systems, or perhaps headphone use. And like the original, the 3D settings were just a distraction here, including the "default" setting. Only the "disengaged" setting seems appropriate.
The buffer measured extremely well. Almost perfect square waves from 20hz-20khz, even when driven far harder than they would ever see in use. The measured output impedance was what is listed as the "corrected" impedance in the specifications, 160 ohms. There was a subtle, but distinct reduction in gain of higher frequencies for the P/P and SET output settings compared to Classic, but no other change to the square wave. That squared with what we were hearing with music. Some of the engineers present had some questions about how the filters are being implemented. Don't know if that's something AMR can share. My assumption when I purchased the original iTube was that it was hybrid, tube input stage, transistor output stage. The iTube 2 specs do say it's a tube output stage, but the engineers wondered how they achieved the low output impedance with a tube output stage, without transistors.
In looking at distortion in the Classic setting, it certainly appears to be a hard clip, typical of an op amp being present.(?) Distortion in all 3 output settings is very low.
Clipping in the SET output does seem more tube like.
I couldn't be happier having the iTube(s) in my system. It is a wonderful match for my system, is dead quiet, and reasonably priced. I could see how it may not be appropriate for all systems, or may not positively influence all systems, but I'd imagine more than not might be improved by adding one. It has excellent measurements, appears well designed and implemented. (Insert thumbs up emoji here)
My system consists of:
Music stored on a NAS, uncompressed FLAC>Sonore SSR renderer>PS Audio Directstream DAC via i2S>First Sound Passive preamp(I still need to pay for)>iTube buffer>split output from the buffer, 1 output to full range amp (Parasound Z Amp or Berning SET) 1 output to an active crossover for the subs>UREI 813C full range, 2x8 cubic foot subs with 2245h JBL 18" woofers.
All analogue, digital, and speaker wires Omega Micro/Mapleshade, except 1 pair of Dueland analogue between DAC and preamp.
