Well, I have finally balanced it...

I replaced my single ended cables with the same make/model of balanced interconnects, Tara Decade and Tara Air-2, between my DAC and preamp, and between my preamp and amp.
-Amp - Accuphase P-450
-Preamp - Pass Labs Aleph P, Gain at 12 o'clock
-DAC - Accuphase DC-61
-Transport - Sony 9000ES
-Speakers - Proac Response 3.8

These are my findings:
-The gain increased by 3dB to 6dB.
-Bass was much fuller and stronger.
-Mids were more natural and smooth.
-Highs had a bit more resolution.
-The sound was a bit quieter, more musical than technical.
It was not, however, as large a difference as I expected. When I do my next cable upgrade, I will likely stay with the balanced setup, probably using Tara The One, which is out of my budget range at the moment. (I know, I know... I need a turntable and tubes!)
Blueswan, your thread is very timely as I have just decided to try the same thing. My amp and CDP are both using truly balanced circuits so I will be trying some XLR cables in January. I will keep you posted to compare results.
Hi Blue
Still running that P450 eh? Glad I found one for my own - wonderful amp to go with my C275. The thing is, neither of these, nor anything else that I'm presently using is truly balanced end-to-end, so although I've wondered about trying the same thing that you've done I can't help but wonder if it's the right way to go? I know - just try it - there's just no other way to find out what'll happen. But the balanced interfaces are "tacked on" requiring the signal to pass through *additional circuitry* which I don't believe could possibly help matters. Maybe I'm wrong though - want to trade cable sets for a couple weeks? :) I've just upgraded to Synergistic's X-series Designers Reference with MCC-III, and I'm certainly wowed! Well anyway, thanks much for sharing your experiences!
Hey Bob
After several days of listening to my balanced cables, I have noticed more air, or separation, between individual instruments and vocals across the soundstage left to right as well as front to back. I did not have this to this degree with single ended. I feel the upgrade to balanced was worth it. How do you like your C-275? I would eventually like to get an Accuphase preamp but my Pass Aleph P with its adjustable gain is going to be difficult to part with.
I'm currently using tara ref gen xlr between an arc amp and preamp, and rca from my cdp. I now have a player with balanced connections (Linn Ikemi) and plan on changing to a balanced cable. Any suggestions as to changing the cdp cable only, or is an upgrade of both cables in order for greater overall improvement? Was I wrong to mix balanced and rca in the first place? Any recommendation of which cable to upgrade to? I have been pretty satisfied with the Tara.
You can usually find Tara Ref Gen XLRs used for less than half list price here at Audiogon. It is OK to use balanced and RCA as you describe (do not try to mix left and right channels this way, nor use both XLR and RCA at the same time though!). If you have the budget and you like Tara, I recommend the Air or Decade series XLRs, also available used at substantial savings.
A "more holographic" stage is also my experience by upgrading to Synergistic's X-series cables, although using all unbalanced interfaces. Actually only the interconnect cables & MCC-III DC control cables are installed so far, as of only about a week ago. HF & LF extension also moved up a notch or two, with an added sense of "naturalness". Just today I received the MK-III DC output shielding modules for the Master Control Center, upgrading it from MK-II to MK-III (when I do actually install them). That will finally complete the X-series upgrade.
Regarding the C275, Blue this preamp is absolutely fantastic & it's not even the latest "C275 V" model. It is an internally unbalanced design (that's OK - most of my equipment is single ended anyway) but it has two channels of balanced line receivers 'tacked on' at the input & two channels of balanced output line driver modules, none of which I've ever used. I also have the internal phono stage which is very good, nice & quiet, high gain, but cartridge-loading options are somewhat limited & Axxis/Accuphase won't provide me a schematic so that I can modify the loading terminations myself, which I'm ticked at them about "being difficult" in that regard. I'm going to go into it anyway after the warranty expires. But the preamp is so completely fully featured & I'm very satisfied nevertheless. Every control feature that my heart desires is included, even has tone & loudness controls for Eq & low-level late night listening, tone-bypass, phase inversion, headphone amp, monitor loops, mono mode, mute mode, remote control of volume & source-selector, and the gold anodized face is a work of art just like the P450 with its big beautifully backlit metering. I went through a lot of equipment trades getting to this point, but when I finally got this stuff I stopped trading, it is just SO GOOD. The C290 I can't even imagine, but it's fully balanced & so the C275 is actually the best fit of their line for my purposes, & actually is the P450's intended mate. For AC cords I use the Synergistic Designer Reference Squared's on both components, also just upgraded to X-series.
Click on my username & scroll down to my Bio if you want to see the complete system description, it's all shown there.