The Mk4 impressed me again on numerous counts, beginning with its dynamic stability. The repeat shifts between piano and full-snot fortissimo tracked effortlessly and giant reserves manifested rapidly out of seemingly nothing when the biggest kettle drums and cello attacks called for it. The fugue-type passages consolidated a different talent between the rather subdued question/answer trades of pizzicato strings and bassoons plus oboes. Whilst Karajan's 1962 reading with the Berlin Philharmonic contains quite a lot of acoustic reverb and the recording is stronger on interpretation than sonic values, all instruments remained clearly located and identified throughout.

Finally I was pleased that regardless of frequency band, this mobilizing of energetic forces occurred without any dips or shifts of the response. This meant that whether my room was flooded by full assault mode or tickled with gentler energies, the aural impression remained unchanged as to its innate balance and coherence (relative to the software of course). The 'Eroica' finale contains rapidly punctuated runs which has the celli hard at work plucking strings and shredding horse hairs. Whether they sung or sawed, they remained clear and uncompromised. Ditto for more lit-up brasses or drums. A faint triangle trail or phat pair of smashed crash cymbals retained unwavering tonality regardless of violence or gentility. Hence a subdued sound remained complete and a really loud one didn't distort.

Which brings me to suspecting that the progression of Mk3 to Mk4 entailed a subtle shift in tonality. Where the treble of the Mk3 had struck me as slightly explicit i.e. a bit north of the zero line, the Mk4 seemed a touch set back or arguably dead-on neutral. In the midband I thought of hearing a tad more micro resolution. Particularly acoustic guitars, voices and violins felt even more accurate and colour intense.

Spatially, the Mk4 cast astonishing depth whilst stage width felt more conservative. "Open House" from Songs for Drella between John Cales and Lou Reed only sports voice and low synth strings plus recurring piano chords which arise precisely centred albeit from the very depth of the virtual venue. Separation struck me as very precise and realistic. Other amps extract more air as bigger walkaround bubbles around the performers. The Mk4 played it a bit more compact but never narrow.

Meanwhile the piano appeared many metres behind my speakers whilst the strings' spread was contained between them. With my Audreal monos, the lateral expanse increases but the piano moves forward toward me. Which was better or more true? I couldn't say except that this difference factored clearly. On this same subject, the Audreal valves express noticeably less dynamic vigour and control in the bass, the low strings appear more subdued and the various decay lengths are fuzzier and less specific. The tube monos also close down sooner on upper harmonics to perpetuate the glowing cliché of the attenuated treble.

Compared to my reference Abacus Ampollo, I saw more overlap on basic tonality. As reported already during my 2013 Mk3 report, the Accustic Arts moved ahead on staying unfazed as complexity ratcheted up. The coincidence of the loud next to the quiet, of massively paralleled instruments occupying shared bandwidth… here the Mk4's virtues kicked in at their full glory to exert superior separation and, relative to the Abacus, the more so as I primed the pump. The Accustic Arts behaved as though it held more headroom reserves. On depth casting, it again gained the additional meter behind the speakers.