Have I Hit The Point Of Diminishing Returns?

You know that it has been written many times before that the latest gear is not necessarily the better sounding gear - most often it's just different sounding gear & at other times it's worse sounding.
You electronics is all good stuff. I'm not a fan of B&W anymore but if you like the sonics then I'd suggest keeping it.
The thing about B&W is that it needs a lot of current to make those speakers come alive. So, from my personal B&W experience, I'm going to say that you are perceiving hitting a ceiling with this gear simply because you do not have sufficient power for the N804. If you can manage it budget-wise I would recommend getting a high current amp - even a class-D power amp. Something with outrageous output power - I'm thinking 400W/ch.
I was never a fan of Cardas cables - too rolled off for me. I believe that this is not helping either. Several inexpensive but supposedly very good sounding brands to try such as Morrow Audio, Signal Cable.
FWIW.

07-27-14: Rlb61
Interesting views. My power amp is 225 wpc dual mono, so I THINK it's powerful enough, but I could be wrong.

For B&W speakers it's not only about Watts/ch - it's about how much current the amplifier can provide to this particular brand of speakers. B&W are well-known to love high-current amplifiers.
I searched hard on the internet to find some A300CR specs but all I could find was A300 & A3CR specs. In particular I was trying to back-calculate the size of the A300CR transformer & the amount of current it could provide.
B&W speakers have a terrible impedance & phase curve in the low freq which gives most amps a lot of trouble because the amp is forced to output a lot more current than what the impedance-only curve informs you.
If you look at purely watts, you might be OK but you probably are shy on output current & that might be limiting you. FWIW.

Thanks Rlb61.
I surfed the web last night again to see if I could get some dope on this amp but it was hard to find anything. So I'm still guessing at the input power transformer size.
Looking at the A300 amp profile I'm thinking it's a 1KVA transformer.
if so, then, I calculate something like 11Amps/channel. Further, you have the plus & minus rails so each rail gets 5.5Amps. That's a very low amount of current to make a floor-standing B&W really sing. You'll get pretty OK sonics using such an amp but the speaker is capable of much more which could be unleashed by using an amp that has a 2KVA or higher power transformer (which would double the current output). Now you are talking of an amp that it pretty tall - like 10" tall - and much more expensive.
Class-D amps get you high current in a smaller chassis & for fairly reasonable price.
Unfortunately, that's the nature of the (B&W) beast.

one serious thing to consider is to passive dual-amp (I'm not using the term bi-amp as bi-amping implies xternal x-over) using a high-current, low-impedance capable, reasonably priced class-D amp to drive the bass (you've biwired them so they have 2 pairs of binding posts). Then, 225W/ch of the A300CR should be plenty for the highs & mids. FWIW.