Krell FPB300 vs 400CX

Hi all

A friend was kind enough to lend me his Cello Duet 350 power amp to try in my system. Compared to my Krell FPB300, the Cello was delightfully grain-free in the upper mids and highs, it was more resolving of inner detail, and there was more space between the instruments. Ultimately I preferred the Krell though, due to its better dynamic contrasts and drive, which make the music more involving. Nevertheless, I'm now aware of a graininess to the the krell, and the way it homogenises the instruments a little. (The Krell is not on a dedicated mains supply BTW, although I hope to install one, one of these days)

Doing the comparison has shown me where I need to go with the next upgrade, i.e keep the dyamics but lose the grain and gain some space.

My Krell dealer can do the internal upgrade from FPB300 to 400CX for about US$2K. My pre is not CAST so the CAST connection is no use to me, but I understand the upgrade replaces some of the input and output stages.

Has anyone upgraded from FPB300 to 400CX and can shed some light on the improvements? I recall Martin Colloms did a comaprison in HFNRR a few years ago but I don't have access to the review.

Any comments and advice would be greatly appreciated.

My system:
Krell 300cd
Krell KRC-HR pre
Krell FPB300
Aerial Acoustics 10T
Interconnects Siltech and Cardas Golden Ref
Spkr Cable Siltech
ESP Essence power cords

Kind regards
Mick