Should I upgrade my Krell FPB 200c to cx300?

I am wondering if someone who has sent their FPB amp back to Krell for the factory upgrade (to the cx version) would tell me if it was worth it. If so, what did it do for the sound (i.e., more bass, control, detail, soundstage or what)?
I can't tell you I have personal experience with the 200. I have had 600c and 650mc and now the 750mcx in my system. I have not sent amps back for upgrade, preferring to purchase them new, though that is irrelevant to this discussion.

Cast mcx is a totally different paradigm for Krell. It loses ALL of the slow, gray, thick, grainy sound Krell has been associated with in the past while maintaining it's traditional strengths. It is extremely neutral tonally, any topend grain is gone. Speed is increased significantly. Now I do not feel need to use Transparent Pixl's or even spikes under my amps to fix those previous issues. This not unsubtle audible difference is immediately heard and is a seachange in the Krell sound. I know from my experience with the 650 to 750 that it's now almost exactly like a cross between the $60k Boulder and the Spectral 360's. Speed, delicacy, smoothness, tonal perfection! It can be delicate and nuanced, unlike previously, then maintain ALL the low end and midrange dynamic magic when needed. Amps have been my own personal grail over the past 5 years, and I've tried, owned and listened carefully to over a dozen of the very best, though by no means not all hiend amps, including Lamm 1.1's, Rowland MC6/8ti, Boulder, Spectral, BelCanto, Audio Research VT100, VT200, Ref300 and Ref600, Atmasphere.

I've always, even before mcx felt that everything else just could not handle the dynamic envelope of live music, and maintain the body and solidity of image like the Krell. Krell makes just about everything else sound broken and allows the suspension of disbelief that allows you to believe you're hearing live music. The only caveat is I have not spent a lot of time with Triode or OTL tubes of the same class, though Atmasphere was very, very good; speakers that I have run have always demanded high current amps, and the heat and maintenance issues have almays made me hesitate. I'm already paranoid enough without having to worry about biasing/tubes etc. I've always felt that the Krells when spiked under transformers and conditioned properly on dedicated 30A lines always sounded to my ears like 600+watt triodes, anyway. This comes in handy also if, like me you have a dual role system, i.e. hi-end home theater.

BTW, if you are running cast, especially the KCT premp, this is a no brainer. Mcx was built by Krell to catch up to the 2nd generation cast output of that preamp. It's very, very good. Mcx also optimizes balanced inputs by speeding up non CAST connections to make them sound much faster than non Mcx non cast connections.

This generation of Krell will cause a lot of people to rethink their previous opinions/bias over Krell products. It should.
just the fact that the new 300 severely devalues the old 200 as far as resale or trade in should be enough to convince you to upgrade.I upgraded my 300 to a 400cx and the amp performs with more authority. everything the previous post mentions is true as i also have the kct pre amp.
How much does the upgrade cost?
I currently have the version of FPB-350M that was made prior to "CAST". I use these with a tube pre-amp, which I do not intend to change. Therefore, does anybody know if these performance improvments (after upgrades) hold true while not usung the "CAST" feature between Krell components?

Well, you actually sort of lose either way. If you don't upgrade, you'll be wondering how much better it would have sounded. If you do upgrade, you're out the cash.

A third alternative is to send the amp to me. This costs less than an upgrade and solves all those nagging problems with voices in your head talking about "CAST" and "dynamic envelopes" and "authority". I certainly wouldn't call you up and talk about that stuff. The amp would also sound better over here than at your house. Let me know.
Mcx also optimizes balanced inputs by speeding up non CAST connections to make them sound much faster than non Mcx non cast connections. Don't sweat the tube pre, I'm sure it sounds fine. You don't need to have all cast to benefit, but when you do it is cumulative. If you are looking for addt'l speed and a slight increase in smoothness on top and midrange, consider it, if not then you won't miss it much, especially if you're already digging what you hear. Out of curiousity, what is your pre?
Thank you all. I appreciate your help. To answer some of the questions listed above, I currently use no preamp. I am running the new Resolution Audio Opus 21 balanced direct to the fpb 200c. I had the KRC-3 and it did a fine job. But the direct route is more revealing and I preferred it in the end. The cost of the upgrade to cx300 is about $1,700.