Hello Philip, I have no experience with the Simaudio 750D. I presume it is
your amplifier, but never having heard it, I won't offer an opinion on how
complementary it will be with the XP-30 pre amp.
I will try to answer your questions about the XP-30 and KX-R noting that I
only heard these two pre amps with the Lamm M1.2 Ref and Pass XA160.5
monoblock amplifiers in a friend's fairly familiar system. He has the Wilson
Sasha speakers with Purist Audio cables. So, my comments should be taken
in that context. They are also based mostly on listening to vinyl, though we
did listen to some digital.
1. Sense of Air: I find this term a bit tough to describe. I will define it as a
sense of palpable space and energy during silent passages which can induce
the anticipation of sound about to arrive. I also think of it as the last ringing
or evidence of a sound hanging in space. With this definition, which preamp
has better air depends on the frequency range. The pass had a bit more air in
the lower FR. Cello, bass, low drums and piano keys seemed slightly more
clear with space and harmonics being more evident with a quieter/blacker
background, especially with the Pass amp. Around the midrange, it was about
even. But with high frequencies, I would give a slight edge to the Ayre. As I
wrote above, there is more HF energy with the Ayre. The tonal balance
is tilted up a bit compared to the Pass. This gives the impression that there is
more lightness and air with cymbals, violin, piano. HF notes are a bit louder,
but they don't ring or resonate more. There seems to be a bit more
information but this might be only an impression because HF is a bit louder.
However, I'm not sure this is more realistic. And I did not miss this extra HF
energy while listening to the Pass.
Transparency: Slight edge to the Pass. This is because of the tonal balance
shift toward the HF with the Ayre. The Pass is more balanced and even
sounding.
Lack of Compression: The Ayre was more open on HF, the Pass more open on
LF and they were about even in the midrange. So during complex orchestral
passages, low level information with drums, cello was a bit more audible and
this gave a better sense of foundation and weight to the music. However,
massed strings, solo violin, right hand piano notes were more emphasized
with the Ayre and this made them pop in the mix a bit more, but again, I'm
not sure this is more realistic than with the Pass, just a bit more energetic and
lively. I should add, when listening to some of the digital, the Ayre was a bit
bright and fatiguing and I would not like it long term. And it did sound
more compressed on digital. The Pass was more relaxed, cleaner and easier
to listen to on digital. There was no sense of fatigue with the Pass.
Well, that was pretty difficult to write and it was based on only one afternoon
of listening. In the end, these differences were really small, especially on
analog, and my suspicion is that cable and amp partnering might make more
of a difference. These are expensive products and you should really try to
hear them in your system context. Overall, I preferred the XP-30, especially
with the Pass amps.
Hornguy's summary above, which is based on hearing these preamps in a
greater range of systems, pretty much matches what I heard. I'll only add that
on certain digital music, the Ayre did sound a bit compressed in the highs and
was slightly fatiguing.