Pros: Low gain, smooth operation, clean & clear sound
Cons: Not for driving low-impedance headphones
Note: this is not a "current" review (as of March 2012) and is mostly a direct copy from post #108 of this full review's thread, which I wrote in September 2010: http://www.head-fi.org/t/511201/review-beyerdynamic-t1-vs-sennheiser-hd800. That forum post has simply been copied here for greater visibility, because it ended up somewhat buried in that review thread as it was posted near the end.
- Plinius CD-101 (CD player) w/ Signal Cable Silver Resolution Reference power cord
- Rockhopper Audio-built M3 as a point of comparison
- BPT IC-SL RCA interconnects (used with Cardas MXLR-FRCA gold adapters to plug into Auditor's XLR inputs)
- Sennheiser HD800 & Beyerdynamic T1 headphones
- Eva Cassidy - Songbird - "Autumn Leaves"
- Julia Fischer - Bach Concertos - Concerto for two violins in D minor - "I. Vivace", "III. Allegro"
- Katie Melua - Piece by Piece - "Shy Boy"
- Laika - Sounds of the Satellites - "Almost Sleeping"
- Priscilla Ahn - A Good Day - "Dream"
- Trifonic - Emergence - "Transgenic"
- Zero 7 - When It Falls - "Home"
Between the M3 and SPL Auditor for the HD800 specifically, I thought the M3 was the better amp and that the SPL was flat out not very good for it. I also ended up concluding that the SPL and T1 paired well together while the M3 and HD800 also paired well together - and that these are probably the ideal pairs from the equipment that I own.
When paired with the SPL, the HD800 consistently revealed a less-than-desirable quality that was sort of hard to pin down exactly - I can't really find the words to express it, but it almost felt like an artificial, synthetic type of sound, most noticeable around vocals and full-range instruments. It almost made music sound like it was being waveguide MIDI-synthesized, not particularly "real" or "physical." I considered this a detraction from the music - but will add that this wasn't an issue on the T1, just the HD800. In contrast there were no such issues on the M3, which had a nice full mid-range, nothing "synthetic" about it. The M3 brought proper body and fullness to instruments and vocals and made them sound "existential" while the SPL tended to make things sound almost "ghostly" in contrast. It could be said that the SPL had a thin, disembodied mid-range compared to the fuller, rich mid-range of the M3, and since the HD800's sound lacks mid-range to an extent (IMO at least), I thought the M3 filled in for this weakness nicely. It definitely added some much-needed balance to the violins on the Julia Fischer CD as they sounded too wispy on the SPL, with not enough substance, but they sounded more natural on the M3 and the "character" of sound from a wooden source (as in, a violin) was correctly conveyed only on the M3.
In the aspect of treble, the SPL's highlighting of details like guitar plucks and sliding made the HD800 sound more treble-tilted than usual, but it wasn't too bad. Some people would probably find it too grating though, and in my case, I thought the resulting treble was not really pleasant to listen to (and I personally like treble in general). Conversely, the SPL had a relative bass sinkhole to match and didn't really help out the HD800 in that aspect. It mostly simply lacked a heavy bass presence in general. The M3, on the other hand, had strong & deep bass, with good impact and power, allowing the HD800 to really assert its bass power. The SPL simply held back too much and did not produce what I thought was a satisfactory level of bass quantity.
Finally, the HD800 clearly showed the soundstage differences between the two amps and revealed a facade behind the SPL - it tended to sound more like it was translating "air" as literal displacement of every layer in the music. Or in other words, the SPL seemed to push everything backwards for the illusion of a deeper soundstage instead of accurately depicting the size. This double-expansion of soundstage with the HD800 (as I also think the HD800 has a larger-than-necessary soundstage) made for distracting listening sessions where it just seemed too over-the-top. Fortunately the M3 recovered a much more accurate soundstage in comparison, smaller in overall depth but it actually seemed to have more width, with a better lateral left-right span that really accentuated the positions of musical elements panned hard left or right.
If I had to sum up the "sound" of each amp, I would describe the SPL Auditor as "pure, colorless, ghostly, synthetic" and the M3 as "vivid, physical, tactile."
I'd recommend the SPL Auditor primarily only for the Beyerdynamic T1 and possibly other ultra-high-impedance headphones (>300 Ohms). As can be inferred from this review, I wouldn't really recommend it for the Senn HD800. For low-impedance, efficient headphones I'd recommend looking elsewhere as this amp does not have the appropriate electrical characteristics to drive such headphones. It was a very poor amp for driving specifically my Audio-Technica ATH-AD2000, Audeze LCD-2 r1, and Grado HP1000.