OK... I've had this for about six weeks now - I've had lots of time to really give this a run out, and I'm still as impress as I was when I got it... my Walkabout hasn't seen the light of day since it landed!
Build quality is absolutely top notch (externally, and fit'n'finish anyhow - I know nowt about electronics and that, but it looks tidy inside). The soft case is practical and well-made.
My initial comment about it picking up a bit more noise than my Walkabout is a non-issue. There's a lot of bottles in it, so if I crank it, and sit facing the amp less than 12" away, it'll hum a bit.,. no surprise there. But in any practical situation - a gig (even a quiet gig) or recording in the house right next to it, it's absolutely quiet.
The DI is beautiful - I've recorded with it, and run the line to the desk to couple shows with big posh PAs, and the engineers have loved it. Hopefully it comes across in some of the clips I've recorded.
And the tone.. it's just 'there'. Clear, rich and authoritative (I'll not say the 'H' word) - the detail I get using flats is fab... the woody growl shines through, and it really responds when you dig in. Keep the input low (lower than you would on most other heads) and it'll stay clean. Crank it a bit, and it'll get a little hairy around the edges, and then you're into fuzz territory.
I've found that the output knob tends to be dialled a little higher than other heads I've used. According to Leszek: 'We use linear potentiometers, other companies use audio potentiometers. If you set the Gain knob and Volume in the center position (12 o'clock) R-400 is actuated on ~55% of the power. In other amps when Gain and Volume are on center, they are actuated ~33%. I prefer use these potentiometers because in my opinion amp is more faster, impulsive.'. The output knob definitely keeps delivering as you get around to the dusty end. I've had the optical compressor light flicker once or twice toward the end of a set, but that was with an 8-ohm cab. Apparently it's just doing it's job. I've got no worries about headroom... but then it's a 400w amp, so there's no reason I should..!
Best amp I've ever owned*. There. I said it. And it's not even expensive.
[size=3]*I don't actually own it yet. Mine is being built, 'cos I want a different colour. Yeah, I know.[/size]