Ok I know the OP is keeping the amp, but these are still fair comments:
the record spot said:
Yep, the Yammy is apparently quite a bright little thing, which puts me right off it. Not sure of the OP's budget, but I'd be inclined to stick on the XTZ Class A D100 - £625 - and Audio Sanctum will refund in full if it's not your thing, they do home demos for XTZ.
I think that 'brightness' in most transistor amps is not a frequency response issue, it's a distortion issue. GNFB (Global negative feedback) can only get rid of the 2nd and 3rd harmonics in general. In a tube amp the rest decay naturally - but in a transistor amp these tend to persist. The problem isn't purely down to the transistors however, it's down to the Class B operation, and crossover distortion. Cross-over distortion is when the +ve going signal crosses over to a different set of transistors for the -ve part of the signal. And vice versa. This is a level independent switching distortion that runs at a few mV, depending upon the amp, bias etc.
The main issue with level independent distortion such as this (same as quiet signals on a CD in fact - quantisation noise is also level independent) is that the distortion is higher (as a %age) when the level is lower.
Given that we make most use of the first watt in an amplifier a powerful transistor amp can have a pretty ugly looking output at 1W, which makes the multiple harmonics sound very 'bright' and 'detailed'. As noted before, GNFB fails to remove the most important (audibly bad) ones.
As the distortion is usually measured at nearly full power this doesn't show up in the figures. Also as THD is the sum of the harmonics, the unpleasant ones are masked by the absence of the harmless 2nd and 3rd. You can learn about this here: http://www.nutshellhifi.com/library/FindingCG.html
The use of THD by magazines has been ruining audio for decades.
Class A fixes a lot of these issues so should be a top priority for any Hi-Fi buff. Class B can sound good with a very clever designer (Naim get around a few of it's nasty sounds) but with a poor designer is sounds just like that Yamaha - harsh and bright.