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

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

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.

My preference is for the Capri directly plugged into the AC, without PC1 Power Factor Correction device. I am planning to test the PC1 on Capri once again in the near future. I have tried 3 PCs on Capri this far:

PAD anniversary -- oddly, I did not like it in this application. . . some unwanted warmth in the lower part of the spectrum, slightly unclear bass, and a feeling of harmonics in the treble region 'disconnected' from the fundamental. Pad Anniv works very well on my X-01 Ltd and on ARC Ref 3 instead.

Cardas Golden Ref -- Good wire without too many flaws. . . not the ultimate extension, and some very slight raggedness at the top.

Shunyata Anaconda helix Alpha -- My preferred PC of the group. . . The most extended and apparently linear. One criticism that has been made by some to Anaconda is to yield slightly softened transients. . . but I still enjoy it very much.

Hope this helps. Guido

Raquel, you are correct, outwardly at least, my preference does not sound terribly credible--the Ref 3 retailing for almost 4X the Capri, being rather diminutive, and internally rather sparsely populated. On the other hand having lost my eyesight 25 years ago, I am not particularly sensitive to the alure of component size, parts count, or striking cosmetics. . . I just let my ears 'do the walking', and more often than not, the 'surprising'.

It is worth pointing out that, in the post you refer, I did not use the term 'best' because it is suggestive of absolute valuations that I personally abhor. . . rather, I said "To my ears, Capri yields greater. . ." What I heard is neither right nor it is wrong. . . but simply my final preference of a comparison which lasted a month and a half in my own system, and admittedly started with a great deal of skepticism from my part. Initially, the Capri being factory fresh, my over 3-year old Ref 3 appeared to yield easily greater refinement, but past the 300 hrs breakin mark, my preference started to shift towards Capri, and firmed up during the subsequent few hundred hours. Approximately 3 months afterward, I put back the Ref 3 into the system for 3 weeks and swapped pres back and forth a few times just to determine if I had suffered temporary delusion. . . even inserted 6H30DRs for the occasion and a fresh SED Winged 6550C. In the end I threw in the towel and I reverted back to Capri. I concluded that either I had not suffered of delusions, or that my delusion was of a more long lasting/grievous nature, because I still easily preferred Capri.

For what it is worth, While I remain uncomfortable at the very word 'best', Rowland has commented several times that Capri is the best sounding Pre he ever created short of Criterion. He attributes a lot of its sound to the Texas Instruments Burr Brown OPA1632 differentially balanced quasi op-amp that he used at Capri's core.

All evaluation was conducted in balanced mode using AudioQuest Sky ICs. For PCs I tried Cardas Golden Ref, Shunyata Anaconda Alpha Helix, and PAD Anniversary.

I Hope this illuminates my experiences with these devices.

G.