Krell FPB 300cx vs Jeff Rowland 501's?

The JRDG 501 are undoubtedly very fine amps to start with. . . loads of controlled power to drive any load and a very open and easy to listen sound. For the reasons explained by Raquel above, if you want the ultimate out of the 501 monos, you need to add to them a pair of external JRDG PC1 active Power Factor Correction (PFC) units. These devices preconvert the AC from the mains to a steady 384V DC current fed into the monos through a banck of intermediate charge capacitors and make the 501 much less sensitive to AC noise and fluctuations. They also keep the 501s internal caps optimilly charged avoiding AC induced ripples. PC1s should yield even larger authority to the 501s and will give them a lot more subtlety and nuance. It is said that the PC1s let the 501s approach the performance of the JRDG 312 stereo flagship, which is my very fav amp of record. The PC1 is not on the jeffrowland.com website yet. Your dealer can order them for you. They list for $1500 each.
I wish I could answer your question more completely and directly. Unfortunately I can't compare the 501s with your Krell, as I have no direct experience with the FPB 300cx . Guido

Thank you all for some great information. It is very helpful. Maybe I should keep the Krell (Raquel, I have a 30 amp dedicated AC line that seems to serve the amp very well).
Maybe, I should ask a follow up question regarding the Krell... How does a Krell FPB 300cx mate with a Jeff Rowland Pre... like the Capri, for example? I would run it balanced.

Chances are the Krell will mate very well with Capri, as Capri has very low output impedance: the manual lists 40 Ohms single ended and 80 Ohms balanced, making it quite compatible with most amps on the market. Other Capri specs and manual at:
http://jeffrowland.com/CapriPreamp.htm
Guido

The Rowland preamps are very quiet, which is a crucial performance parameter, and especially quiet when run with balanced equipment. As Guido notes, they also feature a very low output impedence, meaning they can drive anything (I was able to drive my darTZeel amp with a 50 Ohm BNC connection - my Coherence II has a 50 Ohm output impedence - but ultimately decided on a balanced connection). Finally, the ones I have personal experience with layer space and have the musicality of the best tubed preamps (which says a lot). In short, a Rowland preamp will always be competitive with the best solid-state preamps and would presumably work nicely with the Krell.

I even prefer the JRDG Capri to my ARC Ref 3 for driving my JRDG 312 amp. To my ears, Capri yields greater top to bottom extension, linearity, harmonic complexity, authority, macro and micro dynamics. Staging and imaging seem to me comparable between the two.