Krell FPB 300cx vs FPB400cx

I know there is a power difference but is there a sonic delta between the two? Is the audio signature the same? Are the electronics the same.

Im asking this because I saw an article that indicates the electronics are different.

When I switched amps from Bryston 7BSTs to Krell I initially purhased a 300cx and had that for two days before realizing that, concsistent with my thoughts at that time about upgrading my speakers later, I had sufficient funds and time to return it for full purchase price toward an upgrade to a 400cx. When I got the 400cx home and hooked it up I was not in the least bit disappointed by the increase in clarity and extension driving my Dynaudio Contour 3.0s that I had at that time. There was a marked improvement all around; detail, air, depth, image width and bass extension were the most notable differences as my sonic memory recalls. And later I upgraded to Thiel CS6s and this pairing turned out to be an excellent match (conversations by phone with Shari [a goddess!] at Thiel confirmed that Jim uses Krells routinely for auditioning his designs, especially the cx series). So yes, I experienced first hand the audible improvement of the 400cx over the 300cx. Depending on the remainder of your system, this difference may or may not be so marked as it was in my case. But if you can swing the extra bux, go for the 400cx. And in both cases I was using the KCT pre hooked to the 300cx and 400cx via CAST.
I have a 300c (predecessor to the 400cx, not the 300cx) and have been wondering about the upgrade path for my amp to the 400cx. In my search, I think I read somewhere that the power supply of the 400cx is significantly more substantial than the 300cx. Perhaps this contributes to what Stevecham experienced.
Dbld: Yes I believe that is part of the answer. I also read that the circuit that keeps the amp biased into Class A was also upgraded so that this happens in extrememly short time frames and stays there for 9 minutes before resetting. The toroidal transformer in the 400cx is considerably larger when compared to that of the 300cx. My impression agrees with your suggestion that some of the sonic improvements I heard were due to the "beefier" power supply. Have you talked to Krell about the upgrade? Very friendly folks.
Stevecham: Not yet. I have had a HTS 7.1 for a couple of years now, and have really liked it. It is very much unappreciated in most forums, but I have done some side-by-side comparisons using the two-channel analog pass-through, and it keeps coming out on top.

I just bought my 300c used here on AgoN. I'll need to wait until this busy holiday season calms down before pursuing an upgrade in earnest. If I had better timing, I would have gone for the Audio Pimp's 400cx. It would probably be cheaper for me than upgrading a 300c. So for now I am just kicking the idea around and gathering info (tire-kicking).

I have had occasion to speak with the Krell folks in the past. They have always been very forthright and helpful.