An afternoon of comparing and listening has produced some interesting results.
The Roths have showered themselves in honour to be honest. They seem transparent enough to really show the differences in the two amplifiers, even down to quite subtle details. They have proven themselves to be gregarious partners too. The Rotel RCD965BX has also shown what a superb CD player it is.
The NAD is excellent. Partnered with the Roths it produces a big, dynamic and detailed sound and works well with a wide range of music. It is fluid in the midrange with a slight tendency to over-spotlight this area over the frequency extremes, but it is never less than enjoyable. There is a good sense of the scale of an orchestra and decent sound staging, although on occasion it can be hard to exactly pin-point individual instruments. In rock/pop music it maintains impressive control, only starting to sound slightly compressed when things get really complicated as in the busiest sections of Midlake's 'The Courage of Others', for example.
However, switching to the Pioneer proves that it is, without doubt, the better amplifier. It loses the slight degree of low-end confusion that can affect the NAD and replaces it with a tight and tuneful bass, really gripping the Roths and making them do exactly as they are told. The midrange strength of the Roths continues to shine through but the Pioneer never gives into the slight glossiness that the NAD can be guilty of here. The clarity of voices and instruments is quite spectacular and shows how well designed the little Roths really are. At the treble end things are more open and natural again and there is a tangible sense of atmosphere. All told, it is a very 'live' sound with huge dynamic range and superb levels of detail and the whole thing hangs together beautifully. In orchestral music the Pioneer makes better sense of the contrasting tonal qualities of different instruments and the sound staging is pin sharp and perfectly stable. There is never the slightest sense of compression even when things get very busy and playing the Midlake album is almost like being in the studio with the band. Playing Claudio Arrau's recording of a Brahms Piano Concerto was a revelation with the piano beautifully resolved and the excitement of all the interplay delivered very convincingly.
At this stage I'm not selling either amp, just in case, but on today's evidence it looks like having the Pioneer sent back has had an unexpected benefit!
I'll let you know how things go on.